selftests: kselftest: Make ksft_exit functions return void instead of int

Commit f7d5bcd3 ("selftests: kselftest: Mark functions that
unconditionally call exit() as __noreturn") marked functions that call
exit() as __noreturn but it did not change the return type of these
functions from 'void' to 'int' like it should have (since a noreturn
function by definition cannot return an integer because it does not
return...) because there were many tests that return the result of the
ksft_exit functions, even though it has never been used due to calling
exit().

Now that all uses of 'return ksft_exit...()' have been cleaned up
properly, change the types of the ksft_exit...() functions to void to
match their __noreturn nature.

Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>