lockd: fix server crash on reboot of client holding lock

commit 6e7f90d163 upstream.

I thought I was iterating over the array when actually the iteration is
over the values contained in the array?

Ugh, keep it simple.

Symptoms were a null deference in vfs_lock_file() when an NFSv3 client
that previously held a lock came back up and sent a notify.

Reported-by: Jonathan Woithe <jwoithe@just42.net>
Fixes: 7f024fcd5c ("Keep read and write fds with each nlm_file")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/lockd/svcsubs.c

@@ -179,19 +179,20 @@ nlm_delete_file(struct nlm_file *file)
 static int nlm_unlock_files(struct nlm_file *file)
 {
 	struct file_lock lock;
-	struct file *f;
 
 	lock.fl_type  = F_UNLCK;
 	lock.fl_start = 0;
 	lock.fl_end   = OFFSET_MAX;
-	for (f = file->f_file[0]; f <= file->f_file[1]; f++) {
-		if (f && vfs_lock_file(f, F_SETLK, &lock, NULL) < 0) {
-			pr_warn("lockd: unlock failure in %s:%d\n",
-				__FILE__, __LINE__);
-			return 1;
-		}
-	}
+	if (file->f_file[O_RDONLY] &&
+	    vfs_lock_file(file->f_file[O_RDONLY], F_SETLK, &lock, NULL))
+		goto out_err;
+	if (file->f_file[O_WRONLY] &&
+	    vfs_lock_file(file->f_file[O_WRONLY], F_SETLK, &lock, NULL))
+		goto out_err;
 	return 0;
+out_err:
+	pr_warn("lockd: unlock failure in %s:%d\n", __FILE__, __LINE__);
+	return 1;
 }
 
 /*