gfs2: remove ->writepage

->writepage is only used for single page writeback from memory reclaim,
and not called at all for cgroup writeback.  Follow the lead of XFS
and remove ->writepage and rely entirely on ->writepages.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>