idmapped-mounts: Reset errno to zero before run_test

idmapped-mounts: Reset errno to zero before run_test

If we run case on old kernel that doesn't support mount_setattr and
then fail on our own function before call is_setgid/is_setuid function
to reset errno, run_test will print "Function not implement" error.

We also check whether system support user namespace, so reset errno to
zero after userns check.

Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>