Small tidyup for commit d41a178b.

A comment was left behind claiming that we needed to use malloc() rather
than palloc() because the corresponding free would run in another
thread, but that's not true anymore.  Remove that comment.  And, with
the reason being gone, we might as well actually use palloc().

Back-patch to supported releases, like d41a178b.

Discussion: https://postgr.es/m/CA%2BhUKG%2BpdM9v3Jv4tc2BFx2jh_daY3uzUyAGBhtDkotEQDNPYw%40mail.gmail.com

src/backend/postmaster/postmaster.c

@@ -4836,13 +4836,10 @@ retry:
 
 	/*
 	 * Queue a waiter to signal when this child dies. The wait will be handled
-	 * automatically by an operating system thread pool.
-	 *
-	 * Note: use malloc instead of palloc, since it needs to be thread-safe.
-	 * Struct will be free():d from the callback function that runs on a
-	 * different thread.
+	 * automatically by an operating system thread pool.  The memory will be
+	 * freed by a later call to waitpid().
 	 */
-	childinfo = malloc(sizeof(win32_deadchild_waitinfo));
+	childinfo = palloc(sizeof(win32_deadchild_waitinfo));
 	if (!childinfo)
 		ereport(FATAL,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
@@ -6523,7 +6520,7 @@ waitpid(pid_t pid, int *exitstatus, int options)
 	 * Free struct that was allocated before the call to
 	 * RegisterWaitForSingleObject()
 	 */
-	free(childinfo);
+	pfree(childinfo);
 
 	return pid;
 }