trie_mem_usage() is introduced to calculate the lpm_trie memory usage.
Some small memory allocations are ignored. The inner node is also
ignored.

The result as follows,

- before
10: lpm_trie  flags 0x1
        key 8B  value 8B  max_entries 65536  memlock 1048576B

- after
10: lpm_trie  flags 0x1
        key 8B  value 8B  max_entries 65536  memlock 2291536B
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-3-laoar.shao@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>