selftests: pmtu: detect correct binary to ping ipv6 addresses

Some systems don't have the ping6 binary anymore, and use ping for
everything. Detect the absence of ping6 and try to use ping instead.

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>