Running bwrap as root with custom user namespace with more than 1 uid in uid_map fails

Hello,

for my use case I need to run `bwrap` as root user with a user namespace that has more than 1 uid in uid_map. I've tried to create a user namespace using `unshare`, configure uid_map using `newuidmap` and pass it to `bwrap` through `--userns FD` option. When I run `bwrap` this way though, it fails with the following error:

```
bwrap: Creating newroot failed: Value too large for data type
```

The below script (run as root) can be used to reproduce this issue. It works fine as a regular user.

```shell
#!/bin/sh

user="$(id -un)"
group="$(id -gn)"

# The problem manifests itself only when bwrap is run as root, so warn if script
# is not run as superuser.
if [ $(id -u) -ne 0 ]; then
echo "WARNING: This script needs to be run as root to reproduce the problem with bwrap!"
fi

if ! grep -q "^${user}:" /etc/subuid || ! grep -q "^${group}:" /etc/subgid; then
echo "Missing $user entry in /etc/subuid or /etc/subgid!"
echo "Run as root:"
echo "grep -q '^${user}:' /etc/subuid || echo '${user}:200000:99999' >>/etc/subuid"
echo "grep -q '^${group}:' /etc/subgid || echo '${group}:200000:99999' >>/etc/subgid"
exit 2
fi

# Create a new user namespace in the background with a dummy process just to
# keep it alive.
unshare -U sh -c "sleep 30" &
child_pid=$!

# Set {uid,gid}_map in new user namespace to max allowed range.
# Need to have appropriate entries for user in /etc/subuid and /etc/subgid.
newuidmap $child_pid 0 $(grep "^${user}:" /etc/subuid | cut -d : -f 2- | tr : ' ')
newgidmap $child_pid 0 $(grep "^${group}:" /etc/subgid | cut -d : -f 2- | tr : ' ')

# Tell Bubblewrap to use our user namespace through fd 5.
bwrap \
--ro-bind /bin /bin \
--ro-bind /usr/bin /usr/bin \
--ro-bind /lib /lib \
--ro-bind /usr/lib /usr/lib \
--unshare-ipc \
--unshare-pid \
--unshare-net \
--unshare-uts \
--unshare-cgroup \
--userns 5 \
--cap-add ALL \
--uid 0 \
--gid 0 \
-- \
sh -c 'id' \
5< /proc/$child_pid/ns/user

# bwrap will error out with:
# bwrap: Creating newroot failed: Value too large for data type
```