jfs: Convert insert_metapage() to take a folio

Both of its callers now have a folio, so convert this function.
Use folio_attach_private() instead of manually setting folio->private.
This also gets the expected refcount of the folio correct.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>