selftests: net: py: ensure defer() is only used within a test case

selftests: net: py: ensure defer() is only used within a test case

I wasted a couple of hours recently after accidentally adding
a defer() from within a function which itself was called as
part of defer(). This leads to an infinite loop of defer().
Make sure this cannot happen and raise a helpful exception.

I understand that the pair of _ksft_defer_arm() calls may
not be the most Pythonic way to implement this, but it's
easy enough to understand.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260108225257.2684238-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>