Btrfs: allow for canceling restriper

Implement an ioctl for canceling restriper.  Currently we wait until
relocation of the current block group is finished, in future this can be
done by triggering a commit.  Balance item is deleted and no memory
about the interrupted balance is kept.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/btrfs/ctree.h

@@ -1216,6 +1216,7 @@ struct btrfs_fs_info {
 	struct mutex balance_mutex;
 	atomic_t balance_running;
 	atomic_t balance_pause_req;
+	atomic_t balance_cancel_req;
 	struct btrfs_balance_control *balance_ctl;
 	wait_queue_head_t balance_wait_q;