gfs2: use iomap for buffered I/O in ordered and writeback mode

Switch to using the iomap readpage and writepage helpers for all I/O in
the ordered and writeback modes, and thus eliminate using buffer_heads
for I/O in these cases.  The journaled data mode is left untouched.

(Andreas Gruenbacher: In gfs2_unstuffer_page, switch from mark_buffer_dirty
to set_page_dirty instead of accidentally leaving the page / buffer clean.)

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