seg6: Cleanup duplicates of skb_dst_drop calls

In processing IPv6 segment routing header (SRH), several functions call
skb_dst_drop before ip6_route_input. However, ip6_route_input calls
skb_dst_drop within it, so there is no need to call skb_dst_drop in advance.

Signed-off-by: Yuya Tajima <yuya.tajimaa@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>