ipvlan: avoid spinlock contention in ipvlan_multicast_enqueue()

ipvlan: avoid spinlock contention in ipvlan_multicast_enqueue()

Under high stress, we spend a lot of time cloning skbs,
then acquiring a spinlock, then freeing the clone because
the queue is full.

Add a shortcut to avoid these costs under pressure, as we did
in macvlan with commit 0d5dc1d7aad1 ("macvlan: avoid spinlock
contention in macvlan_broadcast_enqueue()")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260409085238.1122947-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>