xfs: Use struct xfs_bmdr_block instead of struct xfs_btree_block to calculate root node size

The incore data fork of an inode stores the bmap btree root node as 'struct
xfs_btree_block'. However, the ondisk version of the inode stores the bmap
btree root node as a 'struct xfs_bmdr_block'.

xfs_bmap_add_attrfork_btree() checks if the btree root node fits inside the
data fork of the inode. However, it incorrectly uses 'struct xfs_btree_block'
to compute the size of the bmap btree root node. Since size of 'struct
xfs_btree_block' is larger than that of 'struct xfs_bmdr_block',
xfs_bmap_add_attrfork_btree() could end up unnecessarily demoting the current
root node as the child of newly allocated root node.

This commit optimizes space usage by modifying xfs_bmap_add_attrfork_btree()
to use 'struct xfs_bmdr_block' to check if the bmap btree root node fits
inside the data fork of the inode.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/libxfs/xfs_bmap.c

@@ -927,13 +927,15 @@ xfs_bmap_add_attrfork_btree(
 	xfs_inode_t		*ip,		/* incore inode pointer */
 	int			*flags)		/* inode logging flags */
 {
+	struct xfs_btree_block	*block = ip->i_df.if_broot;
 	xfs_btree_cur_t		*cur;		/* btree cursor */
 	int			error;		/* error return value */
 	xfs_mount_t		*mp;		/* file system mount struct */
 	int			stat;		/* newroot status */
 
 	mp = ip->i_mount;
-	if (ip->i_df.if_broot_bytes <= XFS_IFORK_DSIZE(ip))
+
+	if (XFS_BMAP_BMDR_SPACE(block) <= XFS_IFORK_DSIZE(ip))
 		*flags |= XFS_ILOG_DBROOT;
 	else {
 		cur = xfs_bmbt_init_cursor(mp, tp, ip, XFS_DATA_FORK);