selftests: net: veth: make test compatible with dash

veth.sh is a shell script that uses /bin/sh; some distro (Ubuntu for
example) use dash as /bin/sh and in this case the test reports the
following error:

 # ./veth.sh: 21: local: -r: bad variable name
 # ./veth.sh: 21: local: -r: bad variable name

This happens because dash doesn't support the option "-r" with local.

Moreover, in case of missing bpf object, the script is exiting -1, that
is an illegal number for dash:

 exit: Illegal number: -1

Change the script to be compatible both with bash and dash and prevent
the errors above.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>