Lengths, checksum and the DOI are checked.  Checking of the
level and categories are left for the socket layer.

CRC validation is performed in the calipso module to avoid
unconditionally linking crc_ccitt() into ipv6.
Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>