Add a new map ops ->map_mem_usage to print the memory usage of a
bpf map.

This is a preparation for the followup change.
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-2-laoar.shao@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>