gfs2: Use b_folio in gfs2_trans_add_meta()

The lock bit is maintained on the folio, not on the page.  Saves two
calls to compound_head() as well as removing two references to
bh->b_page.

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