NFSD: handle errors better in write_ports_addfd()

If write_ports_add() fails, we shouldn't destroy the serv, unless we had
only just created it.  So if there are any permanent sockets already
attached, leave the serv in place.

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