xfs: remove xfs_bmap_add_extent

There is no real need to the xfs_bmap_add_extent, as the callers
know what kind of extents they need to it.  Removing it means
duplicating the extents to btree conversion logic in three places,
but overall it's still much simpler code and quite a bit less code.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>