md/raid0: Factor out helper for mapping and submitting a bio

Factor out helper function for mapping and submitting a bio out of
raid0_make_request(). We will use it later for submitting both parts of
a split bio.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20230814092720.3931-1-jack@suse.cz
Signed-off-by: Song Liu <song@kernel.org>