SUNRPC: move all of xprt handling into svc_xprt_handle()

svc_xprt_handle() does lots of things itself, but leaves some to the
caller - svc_recv().  This isn't elegant.

Move that code out of svc_recv() into svc_xprt_handle()

Move the calls to svc_xprt_release() from svc_send() and svc_drop()
(the two possible final steps in svc_process()) and from svc_recv() (in
the case where svc_process() wasn't called) into svc_xprt_handle().

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>