[PATCH] dm-raid locking fix

This code was never designed to handle more than one instance of do_work()
running at once.

Signed-Off-By: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>