ipv4: Use standard iovec primitive in raw_probe_proto_opt

The function raw_probe_proto_opt tries to extract the first two
bytes from the user input in order to seed the IPsec lookup for
ICMP packets.  In doing so it's processing iovec by hand and
overcomplicating things.

This patch replaces the manual iovec processing with a call to
memcpy_fromiovecend.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>