ip_frag: fix bit-fields in ipv6 fragment extension

ip_frag: fix bit-fields in ipv6 fragment extension

Previous implementation won't work on every environment. The order of
allocation of bit-fields within a unit (high-order to low-order or
low-order to high-order) is implementation-defined.
Solution: used bytes instead of bit fields.

Signed-off-by: Piotr Azarewicz <piotrx.t.azarewicz@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>