SUNRPC: Ignore return value of ->xpo_sendto

Clean up: All callers of svc_process() ignore its return value, so
svc_process() can safely be converted to return void. Ditto for
svc_send().

The return value of ->xpo_sendto() is now used only as part of a
trace event.

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