treewide: Add SPDX license identifier for missed files

Add SPDX license identifiers to all files which:

 - Have no license information of any form

 - Have EXPORT_.*_SYMBOL_GPL inside which was used in the
   initial scan/conversion to ignore the file

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

  GPL-2.0-only

Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
diff --git a/mm/swapfile.c b/mm/swapfile.c
index cf63b5f..596ac98 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  *  linux/mm/swapfile.c
  *