xfs: fix incorrect extent state in xfs_bmap_add_extent_unwritten_real

There was one spot in xfs_bmap_add_extent_unwritten_real that didn't use the
passed in new extent state but always converted to normal, leading to wrong
behavior when converting from normal to unwritten.

Only found by code inspection, it seems like this code path to move partial
extent from written to unwritten while merging it with the next extent is
rarely exercised.

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