gfs2: Convert gfs2_jhead_pg_srch() to gfs2_jhead_folio_search()

Pass in the folio instead of the page.  Add an assert that this is
not a large folio as we'd need a more complex solution if we wanted to
kmap() each page out of a large folio.  Removes a use of folio->page.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
[agruenba@redhat.com: Rename gfs2_jhead_folio_srch() to gfs2_jhead_folio_search().]
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>