fuse: Convert fuse_writepage_locked to take a folio

The one remaining caller of fuse_writepage_locked() already has a folio,
so convert this function entirely.  Saves a few calls to compound_head()
but no attempt is made to support large folios in this patch.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>