[PATCH] md: make sure the events count in an md array never returns to zero

Now that we sometimes step the array events count backwards (when
transitioning dirty->clean where nothing else interesting has happened - so
that we don't need to write to spares all the time), it is possible for the
event count to return to zero, which is potentially confusing and triggers and
MD_BUG.

We could possibly remove the MD_BUG, but is just as easy, and probably safer,
to make sure we never return to zero.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>