Influx 2 backup fails with panic

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. run influxd in dockercontainer in openshift
2. has a lot of data (problem does not occure with small data)
3. influx backup /some-path

__Expected behavior:__
Backup should succeed

__Actual behavior:__
`h-4.2$ influx backup /backup/test
2021-05-05T09:28:27.556116Z info Backing up KV store {"log_id": "0TvyZKdl000", "path": "/backup/test/20210505T092827Z.bolt"}
panic: invalid freelist page: 0, page type is unknown<00>

goroutine 1 [running]:
go.etcd.io/bbolt.(*freelist).read(0xc000433d00, 0x7fea4a079000)
/home/circleci/go/pkg/mod/go.etcd.io/bbolt@v1.3.5/freelist.go:266 +0x30b
go.etcd.io/bbolt.(*DB).loadFreelist.func1()
/home/circleci/go/pkg/mod/go.etcd.io/bbolt@v1.3.5/db.go:316 +0xd4
sync.(*Once).doSlow(0xc0001ceb68, 0xc00035f468)
/usr/local/go/src/sync/once.go:66 +0xee
sync.(*Once).Do(...)
/usr/local/go/src/sync/once.go:57
go.etcd.io/bbolt.(*DB).loadFreelist(0xc0001cea00)
/home/circleci/go/pkg/mod/go.etcd.io/bbolt@v1.3.5/db.go:309 +0x6c
go.etcd.io/bbolt.Open(0xc00011b9b0, 0x22, 0x180, 0xc00035f5b8, 0x40f7ba, 0x30, 0x19c3c00)
/home/circleci/go/pkg/mod/go.etcd.io/bbolt@v1.3.5/db.go:286 +0x38f
github.com/influxdata/influxdb/v2/bolt.(*KVStore).openDB(0xc0003dfd40, 0xc0004b2960, 0x1af8040)
/home/circleci/go/src/github.com/influxdata/influxdb/bolt/kv.go:93 +0x8b
github.com/influxdata/influxdb/v2/bolt.(*KVStore).Open(0xc0003dfd40, 0x1b1ea00, 0xc00011c000, 0x0, 0x0)
/home/circleci/go/src/github.com/influxdata/influxdb/bolt/kv.go:84 +0x34f
github.com/influxdata/influxdb/v2/backup.RunBackup(0x1b1ea00, 0xc00011c000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x7ffc1e5ac479, 0xc, ...)
/home/circleci/go/src/github.com/influxdata/influxdb/backup/backup.go:69 +0x4bb
main.(*cmdBackupBuilder).backupRunE(0xc000401540, 0xc0003fb8c0, 0xc0004a4740, 0x1, 0x1, 0x0, 0x0)
/home/circleci/go/src/github.com/influxdata/influxdb/cmd/influx/backup.go:98 +0x33d
main.checkSetupRunEMiddleware.func1.1(0xc0003fb8c0, 0xc0004a4740, 0x1, 0x1, 0x0, 0x0)
/home/circleci/go/src/github.com/influxdata/influxdb/cmd/influx/main.go:466 +0x76
github.com/spf13/cobra.(*Command).execute(0xc0003fb8c0, 0xc0004a4720, 0x1, 0x1, 0xc0003fb8c0, 0xc0004a4720)
/home/circleci/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:842 +0x47c
github.com/spf13/cobra.(*Command).ExecuteC(0xc000395600, 0x0, 0x0, 0xc000395600)
/home/circleci/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:950 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
/home/circleci/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:887
main.main()
/home/circleci/go/src/github.com/influxdata/influxdb/cmd/influx/main.go:43 ####+0x58`

__Environment info:__
OpenShift 3.11
InfluxDB 2.0.5 (git: 741389781e) build_date: 2021-04-27T17:57:07Z
Linux 3.10.0-1160.24.1.el7.x86_64 x86_64 (rhel 7)

* System info: Run `uname -srm` and copy the output here
* InfluxDB version: Run `influxd version` and copy the output here
* Other relevant environment details: Container runtime, disk info, etc

__Config:__
Copy any non-default config values here or attach the full config as a gist or file.

__Logs:__
Include snippet of errors in log.

__Performance:__
Generate profiles with the following commands for bugs related to performance, locking, out of memory (OOM), etc.

```sh
# Commands should be run when the bug is actively happening.
# Note: This command will run for ~30 seconds.
curl -o profiles.tar.gz "http://localhost:8086/debug/pprof/all?cpu=30s"
iostat -xd 1 30 > iostat.txt
# Attach the `profiles.tar.gz` and `iostat.txt` output files.
```

Start with backup/backup.go and bolt/kv.go, then trace the influx backup command in cmd/influx/backup.go. Reproduce the failure with a large dataset using the reported command and inspect how the bbolt store is opened. Done means the backup no longer panics and the relevant regression behavior is covered by a test.

Written by the indexing model from the issue text.