Merge branch 'net-memcg-gather-memcg-code-under-config_memcg'

Kuniyuki Iwashima says:

====================
net-memcg: Gather memcg code under CONFIG_MEMCG.

This series converts most sk->sk_memcg access to helper functions
under CONFIG_MEMCG and finally defines sk_memcg under CONFIG_MEMCG.

This is v5 of the series linked below but without core changes
that decoupled memcg and global socket memory accounting.

I will defer the changes to a follow-up series that will use BPF
to store a flag in sk->sk_memcg.

Overview of the series:

  patch 1 is a trivial fix for MPTCP
  patch 2 ~ 9 move sk->sk_memcg accesses to a single place
  patch 10 moves sk_memcg under CONFIG_MEMCG

v4: https://lore.kernel.org/20250814200912.1040628-1-kuniyu@google.com
v3: https://lore.kernel.org/20250812175848.512446-1-kuniyu@google.com
v2: https://lore.kernel.org/20250811173116.2829786-1-kuniyu@google.com
v1: https://lore.kernel.org/20250721203624.3807041-1-kuniyu@google.com
====================

Link: https://patch.msgid.link/20250815201712.1745332-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>