ip6_output: fix regression introduced in r358167 for ipv6 fragmentation

When moving the calculations for the optlen into the if (opt) block
which deals with possible extension headers I failed to initialise
unfragpartlen to the ipv6 header length if there were no extension
headers present. Correct that mistake to make IPv6 fragment length
calculcations work again.

Reported by: hselasky, kp
OKed by: hselasky, kp
MFC after: 3 days
X-MFC with: r358167
PR: 244393