SUNRPC: Move the svc_xdr_recvfrom() tracepoint

Commit c509f15a ("SUNRPC: Split the xdr_buf event class") added
display of the rqst's XID to the svc_xdr_buf_class. However, when
the recvfrom tracepoint fires, rq_xid has yet to be filled in with
the current XID. So it ends up recording the previous XID that was
handled by that svc_rqst.

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