Revert "md-raid: destroy the bitmap after destroying the thread"

This reverts commit e151db8e. Because it
obviously breaks clustered raid as noticed by Neil though it fixed KASAN
issue for dm-raid, let's revert it and fix KASAN issue in next commit.

[1]. https://lore.kernel.org/linux-raid/a6657e08-b6a7-358b-2d2a-0ac37d49d23a@linux.dev/T/#m95ac225cab7409f66c295772483d091084a6d470

Fixes: e151db8e

 ("md-raid: destroy the bitmap after destroying the thread")
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <song@kernel.org>