gfs2: Use b_folio in gfs2_log_write_bh()

We are preparing to remove bh->b_page.  gfs2_log_write() should continue
to operate on pages as some of the memory being logged does not come
from folios, so convert from folio to page in this function.

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