fs: Add missing mutex_unlock

Add a mutex_unlock missing on the error path.  At other exists from the
function that return an error flag, the mutex is unlocked, so do the same
here.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression E1;
@@

* mutex_lock(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* mutex_unlock(E1,...);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/pipe.c

@@ -1169,14 +1169,18 @@ long pipe_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 
 	switch (cmd) {
 	case F_SETPIPE_SZ:
-		if (!capable(CAP_SYS_ADMIN) && arg > pipe_max_pages)
-			return -EINVAL;
+		if (!capable(CAP_SYS_ADMIN) && arg > pipe_max_pages) {
+			ret = -EINVAL;
+			goto out;
+		}
 		/*
 		 * The pipe needs to be at least 2 pages large to
 		 * guarantee POSIX behaviour.
 		 */
-		if (arg < 2)
-			return -EINVAL;
+		if (arg < 2) {
+			ret = -EINVAL;
+			goto out;
+		}
 		ret = pipe_set_size(pipe, arg);
 		break;
 	case F_GETPIPE_SZ:
@@ -1187,6 +1191,7 @@ long pipe_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 		break;
 	}
 
+out:
 	mutex_unlock(&pipe->inode->i_mutex);
 	return ret;
 }