jfs: Convert force_metapage to use a folio

Convert the mp->page to a folio and operate on it.  That lets us
convert metapage_write_one() to take a folio.  Replaces five calls to
compound_head() with one.

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