btrfs: add set_folio_extent_mapped() helper

Turn set_page_extent_mapped() into a wrapper around this version.
Saves a call to compound_head() for callers who already have a folio
and removes a couple of users of page->mapping.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>