xfs: remove the all_bh flag from xfs_convert_page

The all_bh flag is always set when entering the page clustering
machinery with a regular written extent, which means the check for
it is superflous.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>