xfs: xfs_bmap_finish_one should map unwritten extents properly

The deferred bmap work state and the log item can transmit unwritten
state, so the XFS_BMAP_MAP handler must map in extents with that
unwritten state.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>