md/raid5-cache: in r5l_do_submit_io(), submit io->split_bio first

In r5l_do_submit_io(), it is necessary to check io->split_bio before
submit io->current_bio. This is because, endio of current_bio may
free the whole IO unit, and thus change io->split_bio.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>