module: Provide EXPORT_SYMBOL_GPL_FOR_MODULES() helper

Helper macro to more easily limit the export of a symbol to a given
list of modules.

Eg:

  EXPORT_SYMBOL_GPL_FOR_MODULES(preempt_notifier_inc, "kvm");

will limit the use of said function to kvm.ko, any other module trying
to use this symbol will refure to load (and get modpost build
failures).

Requested-by: Masahiro Yamada <masahiroy@kernel.org>
Requested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>