[NET]: Fix race between neigh-timer_handler and neigh_event_send

[NET]: Fix race between neigh-timer_handler and neigh_event_send

Fix a race between neigh_timer_handler() calling down to arp_solicit()
with an sk_buff peeked from the head of the neigh->arp_queue, and
neigh_event_send() unqueuing and freeing the head of the same queue
because it's reached the maximum length of 3, by taking an extra
sk_buff reference while holding neigh->lock.

Signed-off-by: Greg Banks <gnb@melbourne.sgi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>