btrfs: use q which is already obtained from bdev_get_queue

We have already assigned q from bdev_get_queue() so use it.
And rearrange the code for better view.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>