bpf: syscall: Introduce memcg enter/exit helpers

bpf: syscall: Introduce memcg enter/exit helpers

Introduce bpf_map_memcg_enter() and bpf_map_memcg_exit() helpers to
reduce code duplication in memcg context management.

bpf_map_memcg_enter() gets the memcg from the map, sets it as active,
and returns both the previous and the now active memcg.

bpf_map_memcg_exit() restores the previous active memcg and releases the
reference obtained during enter.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102200230.25168-2-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>