IB/rxe: fix GFP_KERNEL in spinlock context

There is skb_clone(skb, GFP_KERNEL) in spinlock context
in rxe_rcv_mcast_pkt().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>