net: sched: em_ipt: set the family based on the packet if it's unspecified

Set the family based on the packet if it's unspecified otherwise
protocol-neutral matches will have wrong information (e.g. NFPROTO_UNSPEC).
In preparation for using NFPROTO_UNSPEC xt matches.

v2: set the nfproto only when unspecified

Suggested-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>