ipmr: Add dedicated mutex for mrt->{mfc_hash,mfc_cache_list}.

ipmr: Add dedicated mutex for mrt->{mfc_hash,mfc_cache_list}.

We will no longer hold RTNL for ipmr_rtm_route() to modify the
MFC hash table.

Only __dev_get_by_index() in rtm_to_ipmr_mfcc() is the RTNL
dependant, otherwise, we just need protection for mrt->mfc_hash
and mrt->mfc_cache_list.

Let's add a new mutex for ipmr_mfc_add(), ipmr_mfc_delete(),
and mroute_clean_tables() (setsockopt(MRT_FLUSH or MRT_DONE)).

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-15-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>