net: add IPv6 header extension parsing function

net: add IPv6 header extension parsing function

Introduce new function for IPv6 header extension parsing able to
determine extension length and next protocol number.

This function is helpful when implementing IPv6 header traversing.

Signed-off-by: Marcin Smoczynski <marcinx.smoczynski@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>
Tested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>