btrfs: use narrower type for btrfs_transaction::num_dirty_bgs

The u64 is an overkill here, we could not possibly create that many
blockgroups in one transaction.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -2894,7 +2894,7 @@ int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
 	struct btrfs_block_rsv *global_rsv;
 	u64 num_heads = trans->transaction->delayed_refs.num_heads_ready;
 	u64 csum_bytes = trans->transaction->delayed_refs.pending_csums;
-	u64 num_dirty_bgs = trans->transaction->num_dirty_bgs;
+	unsigned int num_dirty_bgs = trans->transaction->num_dirty_bgs;
 	u64 num_bytes, num_dirty_bgs_bytes;
 	int ret = 0;
 

fs/btrfs/transaction.h

@@ -70,7 +70,7 @@ struct btrfs_transaction {
 	struct list_head dirty_bgs;
 	struct list_head io_bgs;
 	struct list_head dropped_roots;
-	u64 num_dirty_bgs;
+	unsigned int num_dirty_bgs;
 
 	/*
 	 * we need to make sure block group deletion doesn't race with