IB/srp: Report receive errors correctly

The IB spec does not guarantee that the opcode is available in error
completions.  Hence do not rely on it.  See also commit 948d1e88
("IB/srp: Introduce srp_handle_qp_err()").

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: <stable@vger.kernel.org> # v3.8
Signed-off-by: Roland Dreier <roland@purestorage.com>