ipv6: mcast: Remove unnecessary ASSERT_RTNL and comment.

ipv6: mcast: Remove unnecessary ASSERT_RTNL and comment.

Now, RTNL is not needed for mcast code, and what's commented in
ip6_mc_msfget() is apparent by for_each_pmc_socklock(), which has
lockdep annotation for lock_sock().

Let's remove the comment and ASSERT_RTNL() in ipv6_mc_rejoin_groups().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-11-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>