ext4: fix ext4_ext_direct_IO()'s return value after converting uninit extents

After a direct I/O request covering an uninitalized extent (i.e.,
created using the fallocate system call) or a hole in a file, ext4
will convert the uninitialized extent so it is marked as initialized
by calling ext4_convert_unwritten_extents().  This function returns
zero on success.

This return value was getting returned by ext4_direct_IO(); however
the file system's direct_IO function is supposed to return the number
of bytes read or written on a success.  By returning zero, it confused
the direct I/O code into falling back to buffered I/O unnecessarily.
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -3519,6 +3519,7 @@ retry:
  *
  * This function is called from the direct IO end io call back
  * function, to convert the fallocated extents after IO is completed.
+ * Returns 0 on success.
  */
 int ext4_convert_unwritten_extents(struct inode *inode, loff_t offset,
 				    loff_t len)

fs/ext4/inode.c

@@ -3772,13 +3772,17 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
 		if (ret != -EIOCBQUEUED && ret <= 0 && iocb->private) {
 			ext4_free_io_end(iocb->private);
 			iocb->private = NULL;
-		} else if (ret > 0)
+		} else if (ret > 0) {
+			int err;
 			/*
 			 * for non AIO case, since the IO is already
 			 * completed, we could do the convertion right here
 			 */
-			ret = ext4_convert_unwritten_extents(inode,
-								offset, ret);
+			err = ext4_convert_unwritten_extents(inode,
+							     offset, ret);
+			if (err < 0)
+				ret = err;
+		}
 		return ret;
 	}