net: frags: Add VRF device index to cache and lookup

Fragmentation cache uses information from the IP header to reassemble
packets. That information can be duplicated across VRFs -- same source
and destination addresses, protocol and id. Handle fragmentation with
VRFs by adding the VRF device index to entries in the cache and the
lookup arg.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>