Btrfs: check return value of open_bdev_exclusive properly

Btrfs: check return value of open_bdev_exclusive properly

commit 7f59203abeaf18bf3497b308891f95a4489810ad upstream.

Hit this problem while testing RAID1 failure stuff.  open_bdev_exclusive
returns ERR_PTR(), not NULL.  So change the return value properly.  This
is important if you accidently specify a device that doesn't exist when
trying to add a new device to an array, you will panic the box
dereferencing bdev.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Acked-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>