ipv6: sr: use dst_cache in seg6_input

We already use dst_cache in seg6_output, when handling locally generated
packets. We extend it in seg6_input, to also handle forwarded packets, and avoid
unnecessary fib lookups.

Performances for SRH encapsulation before the patch:
Result: OK: 5656067(c5655678+d388) usec, 5000000 (1000byte,0frags)
  884006pps 7072Mb/sec (7072048000bps) errors: 0

Performances after the patch:
Result: OK: 4774543(c4774084+d459) usec, 5000000 (1000byte,0frags)
  1047220pps 8377Mb/sec (8377760000bps) errors: 0

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>