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.