sched: Un-hide lockdep_tasklist_lock_is_held() for !LOCKDEP

Currently, variables used only within lockdep expressions are flagged as
unused, requiring that these variables' declarations be decorated with
either #ifdef or __maybe_unused.  This results in ugly code.  This commit
therefore causes the lockdep_tasklist_lock_is_held() function to be
visible even when lockdep is not enabled, thus removing the need for
these decorations.  This approach further relies on dead-code elimination
to remove any references to functions or variables that are not available
in non-lockdep kernels.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>