bpf: add percpu stats for bpf_map elements insertions/deletions

bpf: add percpu stats for bpf_map elements insertions/deletions

Add a generic percpu stats for bpf_map elements insertions/deletions in order
to keep track of both, the current (approximate) number of elements in a map
and per-cpu statistics on update/delete operations.

To expose these stats a particular map implementation should initialize the
counter and adjust it as needed using the 'bpf_map_*_elem_count' helpers
provided by this commit.

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Link: https://lore.kernel.org/r/20230706133932.45883-2-aspsk@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>