bpf: bloom_filter memory usage

Introduce a new helper to calculate the bloom_filter memory usage.

The result as follows,
- before
16: bloom_filter  flags 0x0
        key 0B  value 8B  max_entries 65536  memlock 524288B

- after
16: bloom_filter  flags 0x0
        key 0B  value 8B  max_entries 65536  memlock 65856B

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230305124615.12358-9-laoar.shao@gmail.com


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