SUNRPC: Restructure svc_udp_recvfrom()

Clean up. At this point, we are not ready yet to support bio_vecs in
the UDP transport implementation. However, we can clean up
svc_udp_recvfrom() to match the tracing and straight-lining recently
changes made in svc_tcp_recvfrom().

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