gfs2: merge gfs2_writeback_aops and gfs2_ordered_aops

The only difference between the two is that gfs2_ordered_aops sets the
set_page_dirty method to __set_page_dirty_buffers, but given that
__set_page_dirty_buffers is the default, if no method is set, there is no need
to to do that.  Merge the two sets of operations into one.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>