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: NSabrina Dubroca <sd@queasysnail.net>
Acked-by: NStefano Brivio <sbrivio@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>