LSM: SafeSetID: fix userns bug in selftest

LSM: SafeSetID: fix userns bug in selftest

Not sure how this bug got in here but its been there since the original
merge. I think I tested the code on a system that wouldn't let me
clone() with CLONE_NEWUSER flag set so had to comment out these
test_userns invocations.

Trying to map UID 0 inside the userns to UID 0 outside will never work,
even with CAP_SETUID. The code is supposed to test whether we can map
UID 0 in the userns to the UID of the parent process (the one with
CAP_SETUID that is writing the /proc/[pid]/uid_map file).

Signed-off-by: Micah Morton <mortonm@chromium.org>