bpf: hashtab memory usage

bpf: hashtab memory usage

htab_map_mem_usage() is introduced to calculate hashmap memory usage. In
this helper, some small memory allocations are ignore, as their size is
quite small compared with the total size. The inner_map_meta in
hash_of_map is also ignored.

The result for hashtab as follows,

- before this change
1: hash  name count_map  flags 0x1  <<<< no prealloc, fully set
        key 16B  value 24B  max_entries 1048576  memlock 41943040B
2: hash  name count_map  flags 0x1  <<<< no prealloc, none set
        key 16B  value 24B  max_entries 1048576  memlock 41943040B
3: hash  name count_map  flags 0x0  <<<< prealloc
        key 16B  value 24B  max_entries 1048576  memlock 41943040B

The memlock is always a fixed size whatever it is preallocated or
not, and whatever the count of allocated elements is.

- after this change
1: hash  name count_map  flags 0x1    <<<< non prealloc, fully set
        key 16B  value 24B  max_entries 1048576  memlock 117441536B
2: hash  name count_map  flags 0x1    <<<< non prealloc, non set
        key 16B  value 24B  max_entries 1048576  memlock 16778240B
3: hash  name count_map  flags 0x0    <<<< prealloc
        key 16B  value 24B  max_entries 1048576  memlock 109056000B

The memlock now is hashtab actually allocated.

The result for percpu hash map as follows,
- before this change
4: percpu_hash  name count_map  flags 0x0       <<<< prealloc
        key 16B  value 24B  max_entries 1048576  memlock 822083584B
5: percpu_hash  name count_map  flags 0x1       <<<< no prealloc
        key 16B  value 24B  max_entries 1048576  memlock 822083584B

- after this change
4: percpu_hash  name count_map  flags 0x0
        key 16B  value 24B  max_entries 1048576  memlock 897582080B
5: percpu_hash  name count_map  flags 0x1
        key 16B  value 24B  max_entries 1048576  memlock 922748736B

At worst, the difference can be 10x, for example,
- before this change
6: hash  name count_map  flags 0x0
        key 4B  value 4B  max_entries 1048576  memlock 8388608B

- after this change
6: hash  name count_map  flags 0x0
        key 4B  value 4B  max_entries 1048576  memlock 83889408B

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230305124615.12358-4-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>