selftests: netfilter: Ignore tainted kernels in interface stress test

Complain about kernel taint value only if it wasn't set at start
already.

Fixes: 73db1b5d ("selftests: netfilter: Torture nftables netdev hooks")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>