Btrfs: add a flags field to btrfs_transaction

I want to set some per transaction flags, so instead of adding yet another int
lets just convert the current two int indicators to flags and add a flags field
for future use.  Thanks,

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

fs/btrfs/volumes.c

@@ -1462,7 +1462,7 @@ again:
 		btrfs_std_error(root->fs_info, ret,
 			    "Failed to remove dev extent item");
 	} else {
-		trans->transaction->have_free_bgs = 1;
+		set_bit(BTRFS_TRANS_HAVE_FREE_BGS, &trans->transaction->flags);
 	}
 out:
 	btrfs_free_path(path);