net: Define sk_memcg under CONFIG_MEMCG.

Except for sk_clone_lock(), all accesses to sk->sk_memcg
is done under CONFIG_MEMCG.

As a bonus, let's define sk->sk_memcg under CONFIG_MEMCG.

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/20250815201712.1745332-11-kuniyu@google.com


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