ipv6: fix calculation of option len in ip6_append_data

tot_len does specify the size of struct ipv6_txoptions. We need opt_flen +
opt_nflen to calculate the overall length of additional ipv6 extensions.

I found this while auditing the ipv6 output path for a memory corruption
reported by Alexey Preobrazhensky while he fuzzed an instrumented
AddressSanitizer kernel with trinity. This may or may not be the cause
of the original bug.

Fixes: 4df98e76 ("ipv6: pmtudisc setting not respected with UFO/CORK")
Reported-by: Alexey Preobrazhensky <preobr@google.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>