selftests: net: ip_defrag: cover new IPv6 defrag behavior

This patch adds several changes to the ip_defrag selftest, to cover
new IPv6 defrag behavior:

- min IPv6 frag size is now 8 instead of 1280

- new test cases to cover IPv6 defragmentation in nf_conntrack_reasm.c

- new "permissive" mode in negative (overlap) tests: netfilter
sometimes drops invalid packets without passing them to IPv6
underneath, and thus defragmentation sometimes succeeds when
it is expected to fail; so the permissive mode does not fail the
test if the correct reassembled datagram is received instead of a
timeout.

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>