md: simplify code with bio_io_error

Since bio_io_error sets bi_status to BLK_STS_IOERR,
and calls bio_endio, so we can use it directly.

And as mentioned by Shaohua, there are also two
places in raid5.c can use bio_io_error either.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>