xfs: slightly tweak an assert in xfs_fs_map_blocks

We should never see delalloc blocks for a pNFS layout, write or not.
Adjust the assert to check for that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>