IB/rdmavt: restore IRQs on error path in rvt_create_ah()

We need to call spin_unlock_irqrestore() instead of vanilla
spin_unlock() on this error path.

Fixes: 119a8e70

 ("IB/rdmavt: Add AH to rdmavt")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>