selftests: gen_kselftest_tar.sh: Do not clobber kselftest/

The default installation location for gen_kselftest_tar.sh was still
"kselftest/" which collides with the existing directory. Instead, this
moves the installation target into "kselftest_install/kselftest/" and
adjusts the tar creation accordingly. This also adjusts indentation and
logic to be consistent.

Fixes: 42d46e57

 ("selftests: Extract single-test shell logic from lib.mk")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>