ipv4: Simplify iph->daddr overwrite in ip_options_rcv_srr().

We already copy the 4-byte nexthop from the options block into
local variable "nexthop" for the route lookup.

Re-use that variable instead of memcpy()'ing again when assigning
to iph->daddr after the route lookup succeeds.

Signed-off-by: David S. Miller <davem@davemloft.net>