sm-notify: perform DNS lookup in the background.

If an NFS server has no network connectivity when it reboots,
it will block in sm-notify waiting for DNS lookup for a potentially
large number of hosts.  This is not helpful and just annoys the
sysadmin.

So do the DNS lookup in the backgrounded phase of sm-notify,
before sending off the NOTIFY requests.

Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>