samples/pktgen: Use bash as interpreter

These scripts use the non-POSIX 'function' and 'local' keywords so
they won't work with every /bin/sh.  We could drop 'function' as it is
a no-op, but 'local' makes for cleaner scripts.  Require use of bash.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>