nvmet: fix NULL dereference when removing a referral

When item release is called, the parent is already null.  We need the
parent to pass to nvmet_referral_disable so hook it up to
->disconnect_notify.

Reported-by: Tony Asleson <tasleson@redhat.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>