selftests: net: fix setup_ns usage in rtnetlink.sh

selftests: net: fix setup_ns usage in rtnetlink.sh

The setup_ns helper marks the testns global variable as
readonly. Later attempts to set such variable are unsuccessful,
causing a couple test failures.

Avoid completely the variable re-initialization and let the
function access the global value.

Fixes: e9ce7ededf14 ("selftests: rtnetlink: use setup_ns in bonding test")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/6e7c937c8ff73ca52a21a4a536a13a76ec0173a8.1706812005.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>