ipmr/ip6mr: Convert net->ipv[46].ipmr_seq to atomic_t.

ipmr/ip6mr: Convert net->ipv[46].ipmr_seq to atomic_t.

We will no longer hold RTNL for ipmr_mfc_add() and ipmr_mfc_delete().

MFC entry can be loosely connected with VIF by its index for
mrt->vif_table[] (stored in mfc_parent), but the two tables are
not synchronised.  i.e. Even if VIF 1 is removed, MFC for VIF 1
is not automatically removed.

The only field that the MFC/VIF interfaces share is
net->ipv[46].ipmr_seq, which is protected by RTNL.

Adding a new mutex for both just to protect a single field is overkill.

Let's convert the field to atomic_t.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-14-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>