ebtables: among: Support mixed MAC and MAC/IP entries

ebtables: among: Support mixed MAC and MAC/IP entries

Powered by Stefano's support for concatenated ranges, a full among match
replacement can be implemented. The trick is to add MAC-only elements as
a concatenation of MAC and zero-length prefix, i.e. a range from
0.0.0.0 till 255.255.255.255.

Although not quite needed, detection of pure MAC-only matches is left in
place. For those, no implicit 'meta protocol' match is added (which is
required otherwise at least to keep nft output correct) and no concat
type is used for the set.

Signed-off-by: Phil Sutter <phil@nwl.cc>