padata: add SPDX identifier

Add SPDX license identifier according to the type of license text found
in the file.

Cc: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

kernel/padata.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * padata.c - generic interface to process data streams in parallel
  *