netfilter: ipv6: nf_defrag: Kill frag queue on RFC2460 failure

Failures were seen in ICMPv6 fragmentation timeout tests if they were
run after the RFC2460 failure tests. Kernel was not sending out the
ICMPv6 fragment reassembly time exceeded packet after the fragmentation
reassembly timeout of 1 minute had elapsed.

This happened because the frag queue was not released if an error in
IPv6 fragmentation header was detected by RFC2460.

Fixes: 83f1999c ("netfilter: ipv6: nf_defrag: Pass on packets to stack per RFC2460")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>