selftests/net: Improve bind_bhash.sh to accommodate predictable network interface names

Starting with v197, systemd uses predictable interface network names,
the traditional interface naming scheme (eth0) is deprecated, therefore
it cannot be assumed that the eth0 interface exists on the host.

This modification makes the bind_bhash test program run in a separate
network namespace and no longer needs to consider the name of the
network interface on the host.

Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Signed-off-by: David S. Miller <davem@davemloft.net>