netfilter: nft_meta: move bridge meta keys into nft_meta_bridge

Separate bridge meta key from nft_meta to meta_bridge to avoid a
dependency between the bridge module and nft_meta when using the bridge
API available through include/linux/if_bridge.h

Signed-off-by: wenxu <wenxu@ucloud.cn>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>