SUNRPC: Call the default socket callbacks instead of open coding

Rather than code up our own versions of the socket callbacks, just
call the defaults.
This also allows us to merge svc_udp_data_ready() and svc_tcp_data_ready().

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>