ipv6: mcast: Don't hold RTNL for IPV6_DROP_MEMBERSHIP and MCAST_LEAVE_GROUP.

ipv6: mcast: Don't hold RTNL for IPV6_DROP_MEMBERSHIP and MCAST_LEAVE_GROUP.

In __ipv6_sock_mc_drop(), per-socket mld data is protected by lock_sock(),
and only __dev_get_by_index() and __in6_dev_get() require RTNL.

Let's use dev_get_by_index() and in6_dev_get() and drop RTNL for
IPV6_ADD_MEMBERSHIP and MCAST_JOIN_GROUP.

Note that __ipv6_sock_mc_drop() is factorised to reuse in the next patch.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-8-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>