xen/blkback: fix reference counting

If the permission check fails, we drop a reference to the blkif without
having taken it in the first place. The bug was introduced in commit
604c499c (xen/blkback: Check device
permissions before allowing OP_DISCARD).

Cc: stable@vger.kernel.org
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>