rslib: Add SPDX identifiers

The Reed-Solomon library is based on code from Phil Karn who granted
permission to import it into the kernel under the GPL V2.

See commit 15b54237 ("Shared Reed-Solomon ECC library") in the history
git tree at: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

  ...
  The encoder/decoder code is lifted from the GPL'd userspace RS-library
  written by Phil Karn. I modified/wrapped it to provide the different
  functions which we need in the MTD/NAND code.
  ...
Signed-Off-By: Thomas Gleixner <tglx@linutronix.de>
Signed-Off-By: David Woodhouse <dwmw2@infradead.org>
  "No objections at all. Just keep the authorship notices." -- Phil Karn

Add the proper SPDX identifiers according to
Documentation/process/license-rules.rst.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Andrew Morton <akpm@linuxfoundation.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Alasdair Kergon <agk@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/rslib.h

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Generic Reed Solomon encoder / decoder library
  *

lib/reed_solomon/decode_rs.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Generic Reed Solomon encoder / decoder library
  *

lib/reed_solomon/encode_rs.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Generic Reed Solomon encoder / decoder library
  *

lib/reed_solomon/reed_solomon.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Generic Reed Solomon encoder / decoder library
  *