netfilter: ipset: bitmap:ip,mac type requires "src" for MAC

Enforce that the second "src/dst" parameter of the set match and SET target
must be "src", because we have access to the source MAC only in the packet.
The previous behaviour, that the type required the second parameter
but actually ignored the value was counter-intuitive and confusing.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>