netfilter: merge meta_bridge into nft_meta

It overcomplicates things for no reason.
nft_meta_bridge only offers retrieval of bridge port interface name.

Because of this being its own module, we had to export all nft_meta
functions, which we can then make static again (which even reduces
the size of nft_meta -- including bridge port retrieval...):

before:
   text    data     bss     dec     hex filename
   1838     832       0    2670     a6e net/bridge/netfilter/nft_meta_bridge.ko
   6147     936       1    7084    1bac net/netfilter/nft_meta.ko

after:
   5826     936       1    6763    1a6b net/netfilter/nft_meta.ko

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>