IB/srp: Report receive errors correctly

commit cd4e38542a5c2cab94e5410fb17c1cc004a60792 upstream.

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>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>