debugfs: add SPDX identifiers to all debugfs files

It's good to have SPDX identifiers in all files to make it easier to
audit the kernel tree for correct licenses.

Update the debugfs files files with the correct SPDX license identifier
based on the license text in the file itself.  The SPDX identifier is a
legally binding shorthand, which can be used instead of the full boiler
plate text.

This work is based on a script and data from Thomas Gleixner, Philippe
Ombredanne, and Kate Stewart.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/debugfs/file.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  *  file.c - part of debugfs, a tiny little debug file system
  *

fs/debugfs/inode.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  *  inode.c - part of debugfs, a tiny little debug file system
  *

fs/debugfs/internal.h

+// SPDX-License-Identifier: GPL-2.0
 /*
  *  internal.h - declarations internal to debugfs
  *

include/linux/debugfs.h

+// SPDX-License-Identifier: GPL-2.0
 /*
  *  debugfs.h - a tiny little debug file system
  *