btrfs: unwrap folio locking helpers

Another conversion to folio API, use the folio locking directly instead
of back and forth page <-> folio conversions.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>