selftests: net: add in_netns.sh TEST_GEN_PROGS_EXTENDED

Script in_netns.sh is a utility function and not its own test so it
shouldn't be part of the TEST_PROGS. The in_netns.sh get used by
run_afpackettests.
To install in_netns.sh without being added to the main run_kselftest.sh
script use the TEST_GEN_PROGS_EXTENDED variable.

Fixes: 5ff9c1a3

 ("selftests: net: add in_netns.sh to TEST_PROGS")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>