[PATCH] md: ensure bitmap_writeback_daemon handles shutdown properly.

mddev->bitmap gets clearred before the writeback daemon is stopped.  So the
write_back daemon needs to be careful not to dereference the 'bitmap' if it is
NULL.

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>