sunrpc: svc_sock_names should hold ref to socket being closed.

Currently svc_sock_names calls svc_close_xprt on a svc_sock to
which it does not own a reference.
As soon as svc_close_xprt sets XPT_CLOSE, the socket could be
freed by a separate thread (though this is a very unlikely race).

It is safer to hold a reference while calling svc_close_xprt.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>