btrfs: change BUG_ON to assertion in reset_balance_state()

The balance state machine is complex so it's good to verify the
assumptions in helpers, however reset_balance_state() is used
at the end of balance and fs_info::balance_ctl is properly set up before
and protected by the exclusive op ownership in btrfs_balance().
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -3632,7 +3632,7 @@ static void reset_balance_state(struct btrfs_fs_info *fs_info)
 	struct btrfs_balance_control *bctl = fs_info->balance_ctl;
 	int ret;
 
-	BUG_ON(!fs_info->balance_ctl);
+	ASSERT(fs_info->balance_ctl);
 
 	spin_lock(&fs_info->balance_lock);
 	fs_info->balance_ctl = NULL;