net-memcg: Pass struct sock to mem_cgroup_sk_under_memory_pressure().

We will store a flag in the lowest bit of sk->sk_memcg.

Then, we cannot pass the raw pointer to mem_cgroup_under_socket_pressure().

Let's pass struct sock to it and rename the function to match other
functions starting with mem_cgroup_sk_.

Note that the helper is moved to sock.h to use mem_cgroup_from_sk().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://patch.msgid.link/20250815201712.1745332-10-kuniyu@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>