ipv6: mcast: Replace locking comments with lockdep annotations.

ipv6: mcast: Replace locking comments with lockdep annotations.

Commit 63ed8de4be81 ("mld: add mc_lock for protecting per-interface
mld data") added the same comments regarding locking to many functions.

Let's replace the comments with lockdep annotation, which is more helpful.

Note that we just remove the comment for mld_clear_zeros() and
mld_send_cr(), where mc_dereference() is used in the entry of the
function.

While at it, a comment for __ipv6_sock_mc_join() is moved back to the
correct place.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-3-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>