selftests: pmtu: properly redirect stderr to /dev/null

The cleanup function uses "$CMD 2 > /dev/null", which doesn't actually
send stderr to /dev/null, so when the netns doesn't exist, the error
message is shown. Use "2> /dev/null" instead, so that those messages
disappear, as was intended.

Fixes: d1f1b9cb

 ("selftests: net: Introduce first PMTU test")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>