xprtrdma: human-readable completion status

Make it easier to grep the system log for specific error conditions.

The wc.opcode field is not included because opcode numbers are
sparse, and because wc.opcode is not necessarily valid when
completion reports an error.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>