[SCSI] target: iblock/pscsi claim checking for NULL instead of IS_ERR

blkdev_get_by_path() returns an ERR_PTR() or error and it doesn't return
a NULL.  It looks like this bug would be easy to trigger by mistake.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>