bpf: devmap memory usage

A new helper is introduced to calculate the memory usage of devmap and
devmap_hash. The number of dynamically allocated elements are recored
for devmap_hash already, but not for devmap. To track the memory size of
dynamically allocated elements, this patch also count the numbers for
devmap.

The result as follows,
- before
40: devmap  name count_map  flags 0x80
        key 4B  value 4B  max_entries 65536  memlock 524288B
41: devmap_hash  name count_map  flags 0x80
        key 4B  value 4B  max_entries 65536  memlock 524288B

- after
40: devmap  name count_map  flags 0x80  <<<< no elements
        key 4B  value 4B  max_entries 65536  memlock 524608B
41: devmap_hash  name count_map  flags 0x80 <<<< no elements
        key 4B  value 4B  max_entries 65536  memlock 524608B

Note that the number of buckets is same with max_entries for devmap_hash
in this case.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-11-laoar.shao@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>