selftests: pmtu: Reverse return codes of functions

David suggests it's more intuitive to return non-zero on
failures, and zero on success.

No need to introduce tail 'return 0' in functions, they will
return the exit code of the last command anyway.

Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>