[IPV4]: ipip and ip_gre encapsulation bugs

Handling of ipip and ip_gre ICMP error relaying is b0rken; it accesses
8bit field + 3 reserved octets as host-endian 32bit, does comparison,
subtraction and stuffs the result back.  That breaks on big-endian.

Fixed, made endian-clean.

[ Note that this effected code is permanently commented out with
  and ifdef, so this error couldn't actually cause problems for
  anyone. -DaveM ]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>