Add SPDX License identifiers and be honest in MODULE_LICENSE

According to https://docs.kernel.org/process/license-rules.html#id1 "GPL" as parameter to MODULE_LICENSE() means "Module is licensed under GPL version 2". This is not true for this source code which is GPL v3 or later.

So mark the code with the SPDX license identifier GPL-3.0+ and fix the MODULE_LICENSE() macro usages accordingly.

Note that this change enforces license compilance (at least a bit) because some kernel code is only available to modules with a GPL-2.0 compatible license.

This addresses some of the concerns expressed in #137 (closed).