A new helper is introduced to calculate xskmap memory usage.

The xfsmap memory usage can be dynamically changed when we add or remove
a xsk_map_node. Hence we need to track the count of xsk_map_node to get
its memory usage.

The result as follows,
- before
10: xskmap  name count_map  flags 0x0
        key 4B  value 4B  max_entries 65536  memlock 524288B

- after
10: xskmap  name count_map  flags 0x0 <<< no elements case
        key 4B  value 4B  max_entries 65536  memlock 524608B
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-17-laoar.shao@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>