selftests/bpf: Add a test for mmap-able map in map

selftests/bpf: Add a test for mmap-able map in map

Regular BPF hash map is not mmap-able from user space. However, map-in-map
with outer map of type BPF_MAP_TYPE_HASH_OF_MAPS and mmap-able array as
inner map can perform similar operations as a mmap-able hash map. This
can be used by applications that benefit from fast accesses to some local
data.

Add a selftest to show this use case.

Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240723051455.1589192-1-song@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>