btrfs: props: switch prop_handler::extract to struct btrfs_inode

Pass a struct btrfs_inode to the extract() callback as it's an internal
interface, allowing to remove some use of BTRFS_I.

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