garp_pdu_queue() should ways be called with this spin lock.
garp_uninit_applicant() only holds rtnl lock which is not
enough here. A possible race can happen as garp_pdu_rcv()
is called in BH context:
Cc: Eric Dumazet <eric.dumazet@gmail.com> Cc: "David S. Miller" <davem@davemloft.net> Cc: David Ward <david.ward@ll.mit.edu> Cc: "Jorge Boncompte [DTI2]" <jorge@dti2.net> Signed-off-by: Cong Wang <amwang@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>