selftests: net: lib: do not set ns var as readonly

It sounds good to mark the global netns variable as 'readonly', but Bash
doesn't allow the creation of local variables with the same name.

Because it looks like 'readonly' is mainly used here to check if a netns
with that name has already been set, it sounds fine to check if a
variable with this name has already been set instead. By doing that, we
avoid having to modify helpers from MPTCP selftests using the same
variable name as the one used to store the created netns name.

While at it, also avoid an unnecessary call to 'eval' to set a local
variable.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240607-upstream-net-next-20240607-selftests-mptcp-net-lib-v1-3-e36986faac94@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>