jfs: Change metapage->page to metapage->folio

Convert all the users to operate on a folio.  Saves sixteen calls to
compound_head().  We still use sizeof(struct page) in print_hex_dump,
otherwise it will go into the second and third pages of the folio which
won't exist for jfs folios (since they are not large).  This needs a
better solution, but finding it can be postponed.

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