- Better fix for previous error; use DEVOLATILE on the td_lock pointer

it can actually sometimes be something other than sched_lock even on
   schedulers which rely on a global scheduler lock.

Tested by:	kan

sys/kern/sched_4bsd.c

@@ -903,7 +903,7 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
 #endif
 
                 /* I feel sleepy */
-		cpu_switch(td, newtd, &sched_lock);
+		cpu_switch(td, newtd, __DEVOLATILE(struct mtx *, td->td_lock));
 		/*
 		 * Where am I?  What year is it?
 		 * We are in the same thread that went to sleep above,

sys/kern/sched_ule.c

@@ -1487,7 +1487,7 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
 			PMC_SWITCH_CONTEXT(td, PMC_FN_CSW_OUT);
 #endif
 
-		cpu_switch(td, newtd, &sched_lock);
+		cpu_switch(td, newtd, __DEVOLATILE(struct mtx *, td->td_lock));
 #ifdef	HWPMC_HOOKS
 		if (PMC_PROC_IS_USING_PMCS(td->td_proc))
 			PMC_SWITCH_CONTEXT(td, PMC_FN_CSW_IN);