selftests: netfilter: fix conntrack_dump_flush retval on unsupported kernel

selftests: netfilter: fix conntrack_dump_flush retval on unsupported kernel

With CONFIG_NETFILTER=n test passes instead of skip.  Before:

 ./run_kselftest.sh -t net/netfilter:conntrack_dump_flush
[..]
 # Starting 3 tests from 1 test cases.
 #  RUN           conntrack_dump_flush.test_dump_by_zone ...
 mnl_socket_open: Protocol not supported
[..]
 ok 3 conntrack_dump_flush.test_flush_by_zone_default
 # PASSED: 3 / 3 tests passed.
 # Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0

After:
  mnl_socket_open: Protocol not supported
[..]
  ok 3 conntrack_dump_flush.test_flush_by_zone_default # SKIP cannot open netlink_netfilter socket
  # PASSED: 3 / 3 tests passed.
  # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:3 error:0

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240422103358.3511-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>