Driver core: Fix up build when CONFIG_BLOCK=N

This fixes up the driver core build errors when CONFIG_BLOCK=N

Thanks to Alexander van Heukelum <heukelum@mailshack.com> for the basis
of this patch, and to Jeremy Fitzhardinge <jeremy@goop.org> for
reporting the problem.


Cc: Alexander van Heukelum <heukelum@mailshack.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>