net/ipv6: Fix IPv6 fragment nexthdr field in case of no option headers

With or without options headers it has to work. Currently it was
setting always hop-by-hop next header which is obviously wrong but
worked on ipv6_fragment test since that one has only packets with
optional headers (hop-by-hop in that case has to be the first optional
header).

Fixes #14622

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>