ebtables: Fix among match

ebtables: Fix among match

Fixed commit broke among match in two ways:

1) The two lookup sizes are 12 and 6, not 12 and 4 - among supports
   either ether+IP or ether only, not IP only.

2) Adding two to sreg_count to get the second register is too simple: It
   works only for four byte regs, not the 16 byte ones. The first
   register is always a 16 byte one, though.

Fixing (1) is trivial, fix (2) by introduction of nft_get_next_reg()
doing the right thing. For consistency, use it for among match creation,
too.

Fixes: f315af1cf8871 ("nft: track each register individually")
Signed-off-by: Phil Sutter <phil@nwl.cc>