close_range/closefrom: fix regression from close_range introduction

close_range will clamp the range between [0, fdp->fd_lastfile], but failed
to take into account that fdp->fd_lastfile can become -1 if all fds are
closed. =-( In this scenario, just return because there's nothing further we
can do at the moment.

Add a test case for this, fork() and simply closefrom(0) twice in the child;
on the second invocation, fdp->fd_lastfile == -1 and will trigger a panic
before this change.

X-MFC-With:	r359836

sys/kern/kern_descrip.c

@@ -1333,6 +1333,14 @@ kern_close_range(struct thread *td, u_int lowfd, u_int highfd)
 		ret = EINVAL;
 		goto out;
 	}
+
+	/*
+	 * If fdp->fd_lastfile == -1, we're dealing with either a fresh file
+	 * table or one in which every fd has been closed.  Just return
+	 * successful; there's nothing left to do.
+	 */
+	if (fdp->fd_lastfile == -1)
+		goto out;
 	/* Clamped to [lowfd, fd_lastfile] */
 	highfd = MIN(highfd, fdp->fd_lastfile);
 	for (fd = lowfd; fd <= highfd; fd++) {

tests/sys/file/closefrom_test.c

@@ -146,7 +146,7 @@ main(void)
 	pid_t pid;
 	int fd, i, start;
 
-	printf("1..19\n");
+	printf("1..20\n");
 
 	/* We better start up with fd's 0, 1, and 2 open. */
 	start = devnull();
@@ -309,5 +309,21 @@ main(void)
 		fail("close_range", "highest fd %d", fd);
 	ok("close_range");
 
+	/* Fork a child process to test closefrom(0) twice. */
+	pid = fork();
+	if (pid < 0)
+		fail_err("fork");
+	if (pid == 0) {
+		/* Child. */
+		closefrom(0);
+		closefrom(0);
+		cok(info, "closefrom(0)");
+	}
+	if (wait(NULL) < 0)
+		fail_err("wait");
+	if (info->failed)
+		fail(info->tag, "%s", info->message);
+	ok(info->tag);
+
 	return (0);
 }