Can't create BPF_LPM_TRIE (1 EINVAL)

### System Info
**Distro**: Fedora 31 server
**Linux kernel**: 5.6.2
**LLVM**: 9.0.1 (Fedora 9.0.1-2.fc31)
**Python**: 3.7.6
**BCC**: 0.13.0-495a4a34 (from master, but same issue from Fedora 31 package)

Everything is run as root, SELinux is disabled, and ulimits (soft/hard) are set to unlimited.

### Problem
I cannot create a `BPF_MAP_TYPE_LPM_TRIE` map using BCC, but I can create it fine in-tree.

None of the following work with BCC.
```
BPF_LPM_TRIE(lpm_map);
BPF_F_TABLE("lpm_trie", long, long, lpm_map, 10240, BPF_F_NO_PREALLOC);
BPF_F_TABLE("lpm_trie", long, long, lpm_map, 8, BPF_F_NO_PREALLOC);
```

This works fine without BCC.
```
struct bpf_map_def SEC("maps") trie = {
.type = BPF_MAP_TYPE_LPM_TRIE,
.key_size = sizeof(long),
.value_size = sizeof(long),
.max_entries = 10000,
.map_flags = BPF_F_NO_PREALLOC
};
```

### Error output
This shows 2 successful (BPF_ARRAY) maps being created before failing (twice?) at creating a BPF_LPM_TRIE map.

```
Using BCC version 0.13.0-495a4a34
bpf(BPF_BTF_LOAD, {btf="\237\353\1\0\30\0\0\0\0\0\0\0\260\10\0\0\260\10\0\0j\f\0\0\1\0\0\0\2\0\0\4"..., btf_log_buf=NULL, btf_size=5426, btf_log_size=0, btf_log_level=0}, 120) = 3
bpf(BPF_MAP_CREATE, {map_type=BPF_MAP_TYPE_ARRAY, key_size=4, value_size=4, max_entries=8, map_flags=0, inner_map_fd=0, map_name="test_map", map_ifindex=0, btf_fd=3, btf_key_type_id=7, btf_value_type_id=7}, 120) = 4
bpf(BPF_MAP_CREATE, {map_type=BPF_MAP_TYPE_ARRAY, key_size=4, value_size=8, max_entries=256, map_flags=0, inner_map_fd=0, map_name="xdp_stats_map", map_ifindex=0, btf_fd=3, btf_key_type_id=7, btf_value_type_id=8}, 120) = 5
bpf(BPF_MAP_CREATE, {map_type=BPF_MAP_TYPE_LPM_TRIE, key_size=8, value_size=8, max_entries=10240, map_flags=BPF_F_NO_PREALLOC, inner_map_fd=0, map_name="lpm_map", map_ifindex=0, btf_fd=3, btf_key_type_id=2, btf_value_type_id=2}, 120) = -1 EINVAL (Invalid argument)
bpf(BPF_MAP_CREATE, {map_type=BPF_MAP_TYPE_LPM_TRIE, key_size=8, value_size=8, max_entries=10240, map_flags=BPF_F_NO_PREALLOC, inner_map_fd=0, map_name="", map_ifindex=0, btf_fd=3, btf_key_type_id=2, btf_value_type_id=2}, 120) = -1 EINVAL (Invalid argument)
could not open bpf map: lpm_map, error: Invalid argument
Traceback (most recent call last):
File "./src/examples/drop-icmp/user.py", line 110, in
main()
File "./src/examples/drop-icmp/user.py", line 73, in main
debug=args.debug)
File "./src/examples/drop-icmp/user.py", line 22, in load_xdp_program
debug=debug)
File "/usr/lib/python3.7/site-packages/bcc/__init__.py", line 356, in __init__
raise Exception("Failed to compile BPF module %s" % (src_file or ""))
Exception: Failed to compile BPF module b'/root/ebpf/src/examples/drop-icmp/kern.c'
+++ exited with 1 +++
```

Compared to a successful map create from in-tree BPF, I'm not seeing much difference in the syscall itself... Here's the working syscall without BCC.

```
bpf(BPF_MAP_CREATE, {map_type=BPF_MAP_TYPE_LPM_TRIE, key_size=8, value_size=8, max_entries=10000, map_flags=BPF_F_NO_PREALLOC, inner_map_fd=0, map_name="trie", map_ifindex=0, btf_fd=0, btf_key_type_id=0, btf_value_type_id=0}, 120) = 5
```

The in-tree sanity checks are being passed, but I'm not sure what BCC is doing differently. The in-tree checks are found here - https://elixir.bootlin.com/linux/v5.6.2/source/kernel/bpf/lpm_trie.c#L549 . Can someone point me to where the sanity checks for this map type happen in BCC source?

Start with src/examples/drop-icmp/kern.c and src/examples/drop-icmp/user.py, then compare BCC's generated BPF_MAP_CREATE calls with the working in-tree syscall shown in the report. Inspect the BCC map creation path and the Linux v5.6.2 lpm_trie.c sanity checks linked in the issue. Done means identifying the BCC-specific incompatibility and making LPM_TRIE creation work without regressing the existing BPF_ARRAY maps.

Written by the indexing model from the issue text.