xprtrdma: Clean up Receive trace points

For clarity, report the posting and completion of Receive CQEs.

Also, the wc->byte_len field contains garbage if wc->status is
non-zero, and the vendor error field contains garbage if wc->status
is zero. For readability, don't save those fields in those cases.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>