net: ipv6: rename nexthdr and next_nexthdr for readability

The current names are confusing. Indeed "nexthdr" if the type of the
header currently processed, while "next_nexthdr" is the nexthdr field of
the current header.

Rename them to improve readability and make it less error-prone.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>