bpf: reuseport_array memory usage

bpf: reuseport_array memory usage

A new helper is introduced to calculate reuseport_array memory usage.

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

- after
14: reuseport_sockarray  name count_map  flags 0x0
        key 4B  value 8B  max_entries 65536  memlock 524544B

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-7-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>