kcsan: Clean up the main KCSAN Kconfig option

This patch cleans up the rules of the 'KCSAN' Kconfig option by:

  1. implicitly selecting 'STACKTRACE' instead of depending on it;
  2. depending on DEBUG_KERNEL, to avoid accidentally turning KCSAN on if
     the kernel is not meant to be a debug kernel;
  3. updating the short and long summaries.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>