SUNRPC: change svc_get() to return the svc.

It is common for 'get' functions to return the object that was 'got',
and there are a couple of places where users of svc_get() would be a
little simpler if svc_get() did that.

Make it so.

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