Force a commit on kern_mutex.c to explain reason for last commit but while

I'm at it also add a comment in mtx_validate() explaining the purpose
of the last change.

Basically, this fixes booting kernels compiled with MUTEX_DEBUG. What used
to happen is before we setidt from init386() [still using BTX idt], we
called mtx_init() on several mutex locks, notably Giant and some others.
This is a problem for MUTEX_DEBUG because it enables mtx_validate() which
calls kernacc(), some of which in turn requires Giant.
Fix by calling kernacc() from mtx_validate() only if (!cold).

sys/kern/kern_mutex.c

@@ -611,6 +611,11 @@ mtx_validate(struct mtx *m)
  * we can re-enable the kernacc() checks.
  */
 #ifndef __alpha__
+	/*
+	 * Can't call kernacc() from early init386(), especially when
+	 * initializing Giant mutex, because some stuff in kernacc()
+	 * requires Giant itself.
+	 */ 
 	if (!cold)
 		if (!kernacc((caddr_t)m, sizeof(m),
 		    VM_PROT_READ | VM_PROT_WRITE))

sys/kern/subr_turnstile.c

@@ -611,6 +611,11 @@ mtx_validate(struct mtx *m)
  * we can re-enable the kernacc() checks.
  */
 #ifndef __alpha__
+	/*
+	 * Can't call kernacc() from early init386(), especially when
+	 * initializing Giant mutex, because some stuff in kernacc()
+	 * requires Giant itself.
+	 */ 
 	if (!cold)
 		if (!kernacc((caddr_t)m, sizeof(m),
 		    VM_PROT_READ | VM_PROT_WRITE))