net: ipv6: Fix the constness of function parameters

Some of the IPv6 utility functions were missing const in
one of the parameters that are not modified by the corresponding
function.

Change-Id: Ic9fe53daac288570c14423fd9410dcf15d1c5cfa
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>