sfc: offload foreign RHS rules without an encap match

Normally, if a TC filter on a tunnel netdev does not match on any
 encap fields, we decline to offload it, as it cannot meet our
 requirement for a <sip,dip,dport> tuple for the encap match.
However, if the rule has a nonzero chain_index, then for a packet to
 reach the rule, it must already have matched a LHS rule which will
 have included an encap match and determined the tunnel type, so in
 that case we can offload the right-hand-side rule.

Reviewed-by: Pieter Jansen van Vuuren <pieter.jansen-van-vuuren@amd.com>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>