[PATCH] md: make sure bitmap_daemon_work actually does work.

The 'lastrun' time wasn't being initialised, so it could be half a
jiffie-cycle before it seemed to be time to do work again.

Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>