gfs2: Convert gfs2_end_log_write_bh() to work on a folio

gfs2: Convert gfs2_end_log_write_bh() to work on a folio

gfs2_end_log_write() has to handle bios which consist of both pages
which belong to folios and pages which were allocated from a mempool and
do not belong to a folio.  It would be cleaner to have separate endio
handlers which handle each type, but it's not clear to me whether that's
even possible.

This patch is slightly forward-looking in that page_folio() cannot
currently return NULL, but it will return NULL in the future for pages
which do not belong to a folio.

This was the last user of page_has_buffers(), so remove it.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>