md: check level before create and exit io_acct_set
The bio_set (io_acct_set) is used by personalities to clone bio and
trace the timestamp of bio. Some personalities such as raid1/10 don't
need the bio_set, so add check to not create it unconditionally.
Also update the comment for md_account_bio to make it more clear.
Suggested-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Guoqing Jiang <jiangguoqing@kylinos.cn> Signed-off-by: Song Liu <song@kernel.org>