Driver core: let request_module() send a /sys/modules/kmod/-uevent

On recent systems, calls to /sbin/modprobe are handled by udev depending
on the kind of device the kernel has discovered. This patch creates an
uevent for the kernels internal request_module(), to let udev take control
over the request, instead of forking the binary directly by the kernel.
The direct execution of /sbin/modprobe can be disabled by setting:
  /sys/module/kmod/mod_request_helper (/proc/sys/kernel/modprobe)
to an empty string, the same way /proc/sys/kernel/hotplug is disabled on an
udev system.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>