net: warn, if pf->create does not clear sock->sk on error

All pf->create implementations have been fixed now to clear sock->sk on
error, when they deallocate the allocated sk object.

Put a warning in place to make sure we don't break this promise in the
future.

Suggested-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241014153808.51894-9-ignat@cloudflare.com


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