sunrpc: Simplify trace_svc_recv

There doesn't seem to be a lot of value in calling trace_svc_recv
in the failing case.

1. There are two very common cases: one is the transport is not
ready, and the other is shutdown. Neither is terribly interesting.

2. The trace record for the failing case contains nothing but
the status code.

Therefore the trace point call site in the error exit is removed.
Since the trace point is now recording a length instead of a
status, rename the status field and remove the case that records a
zero XID.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>