generate "change" uevent for loop device

Recent udev versions probe loop devices for filesystems meaning that
the /dev/disk hierarchy may contain useful entries such as

 $ ls -l /dev/disk/by-label/Fedora-12-x86_64-Live
 lrwxrwxrwx 1 root root 11 Mar 11 13:41 /dev/disk/by-label/Fedora-12-x86_64-Live -> ../../loop0

Unfortunately, no "change" uevent is generated when the loop device is
detached so the symlink persists. Additionally, no "change" uevent is
guaranteed to be generated when attaching an fd or changing capacity.
For example,  user space could open the loop device O_RDONLY (in fact,
recent util-linux-ng does this) so udev's OPTIONS+="watch" machinery may
not trigger the "change" uevent.

This patch ensures that the "change" uevent is generated in all of
these cases. As a result, the /dev/disk hierarchy works as expected
for loop devices.

Signed-off-by: David Zeuthen <davidz@redhat.com>
Acked-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>