ip6mr does not have rtnetlink interface for MFC unlike ipmr,
which uses dev_get_by_index_rcu() to set struct mfcctl.mfcc_parent.
ip6mr_mfc_add() and ip6mr_mfc_delete() are called under RTNL
from ip6_mroute_setsockopt() only.
There are no RTNL dependant, but ip6_mroute_setsockopt() reuses
RTNL just for mrt->mfc_hash and mrt->mfc_cache_list.
Let's replace RTNL with a new per-netns mutex.
Later, ip6mr_notifier_ops and ipmr_seq will be moved under
CONFIG_IPV6_MROUTE.
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260604224712.3209821-15-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
a4c9842172