Make sure to clear the driverfs_dev pointer when we do del_gendisk() (on
disk removal), so that other users that may still have a ref to the disk
won't try to use the stale pointer.

Also move the KOBJ_REMOVE uevent handler up, so that the uevent still
has access to the driverfs_dev data.

This all should hopefully fix the problems with MMC umounts after device
removals that caused commit 56cf6504 and
its reversal (1a2acc9e).

Original problem reported by Todd Blumer and others.
Acked-by: Greg KH <gregkh@suse.de>
Cc: Russell King <rmk+lkml@arm.linux.org.uk>
Cc: James Bottomley <James.Bottomley@SteelEye.com>
Cc: Erik Mouw <erik@harddisk-recovery.com>
Cc: Andrew Vasquez <andrew.vasquez@qlogic.com>
Cc: Todd Blumer <todd@sdgsystems.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>