gfs2: Convert gfs2_find_jhead() to use a folio

Remove a call to grab_cache_page() by using a folio throughout
this function.

[agruenba@redhat.com: Adjust to return value difference between
bio_add_page() and bio_add_folio().]

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