i40e: Fix memory leak at failure path in i40e_dbg_command_write()

The patch fixes a leak of 'cmd_buf' when copy_from_user() failed
in i40e_dbg_command_write().

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_debugfs.c

@@ -989,8 +989,10 @@ static ssize_t i40e_dbg_command_write(struct file *filp,
 	if (!cmd_buf)
 		return count;
 	bytes_not_copied = copy_from_user(cmd_buf, buffer, count);
-	if (bytes_not_copied < 0)
+	if (bytes_not_copied < 0) {
+		kfree(cmd_buf);
 		return bytes_not_copied;
+	}
 	if (bytes_not_copied > 0)
 		count -= bytes_not_copied;
 	cmd_buf[count] = '\0';