IB/mlx4: Don't call dma_free_coherent() with irqs disabled

mlx4_ib_free_cq_buf() should not be called under spin_lock_irq() since
it calls dma_free_coherent(), which needs irqs enabled.  Fix this by
deferring the free to outside the locked region.

This was found due to the

	WARN_ON(irqs_disabled());

in swiotlb_free_coherent().

Signed-off-by: Vladimir Sokolovsky <vlad@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>