xfs: fix typo in comment of xfs_dinode_t.

There should be "XFS_DFORK_DPTR, XFS_DFORK_APTR, and XFS_DFORK_PTR" instead
of "XFS_DFORK_PTR, XFS_DFORK_DPTR, and XFS_DFORK_PTR".

Signed-off-by: Chen Baozi <baozich@gmail.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>