Btrfs: set FMODE_EXCL in btrfs_device->mode

This fixes a bug introduced in d4d77629, where the device added online
(and therefore initialized via btrfs_init_new_device()) would be left
with the positive bdev->bd_holders after unmount.  Since d4d77629 we no
longer OR FMODE_EXCL explicitly on blkdev_put(), set it in
btrfs_device->mode.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>