xfs: remove attr fork handling in xfs_bmap_finish_one

We never do COW operations for the attr fork, so don't pretend we handle
them.

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>