Btrfs: reserve space for block groups

This changes our delayed refs calculations to include the space needed
to write back dirty block groups.

Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/transaction.h

@@ -64,6 +64,7 @@ struct btrfs_transaction {
 	struct list_head pending_ordered;
 	struct list_head switch_commits;
 	struct list_head dirty_bgs;
+	u64 num_dirty_bgs;
 	spinlock_t dirty_bgs_lock;
 	struct btrfs_delayed_ref_root delayed_refs;
 	int aborted;