Instead of defining a local version of struct iphdr use the standard
definition from <linux/ip.h>.

Several fields in the <linux/ip.h> definition have different names:
 - proto -> protocol
 - src -> saddr
 - dest -> daddr
 - total_length -> tot_len
 - checksum -> check

Also, 'ver_len' is composed by 'version' and 'ihl' in <linux/ip.h>.
Signed-off-by: NLuis Henriques <luis.henriques@canonical.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>