btrfs: convert clear_page_extent_mapped() to take a folio

The old page API is being gradually replaced and converted to use folio
to improve code readability and avoid repeated conversion between page
and folio. Now clear_page_extent_mapped() can deal with a folio
directly, so change its name to clear_folio_extent_mapped().

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>