We weren't actually waking up the md thread after setting
MD_RECOVERY_NEEDED when assembling an array, so it is possible to lose a
race and not actually start resync.

So add a call to md_wakeup_thread, and while we are at it, remove all the
"if (mddev->thread)" guards as md_wake_thread does its own checking.
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>