i.e.  missing or failed drives are moved to the end of the list.  The means
a 3 drive md array with the first drive missing can be shrunk to a two
drive array.  Currently that isn't possible.

Also, the "last_used" device number might be out-of-range after the number
of devices is reduced, so we set it to 0.
Signed-off-by: NNeil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>