[PATCH] md: send online/offline uevents when an md array starts/stops

This allows udev to do something intelligent when an array becomes
available.

Acked-by: Greg KH <greg@kroah.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>