xen/scsiback: correct frontend counting

xen/scsiback: correct frontend counting

When adding a new frontend to xen-scsiback don't decrement the number
of active frontends in case of no error. Doing so results in a failure
when trying to remove the xen-pvscsi nexus even if no domain is using
it.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit f285aa8db7cc4432c1a03f8b55ff34fe96317c11)
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>