gfs2: stop using generic_writepages in gfs2_ail1_start_one

Use filemap_fdatawrite_wbc instead of generic_writepages in
gfs2_ail1_start_one so that the functin can also cope with address_space
operations that only implement ->writepages and to properly account
for cgroup writeback.

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