bpf: stackmap memory usage

A new helper is introduced to get stackmap memory usage. Some small
memory allocations are ignored as their memory size is quite small
compared to the totol usage.

The result as follows,
- before
16: stack_trace  name count_map  flags 0x0
        key 4B  value 8B  max_entries 65536  memlock 1048576B

- after
16: stack_trace  name count_map  flags 0x0
        key 4B  value 8B  max_entries 65536  memlock 2097472B

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-6-laoar.shao@gmail.com


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