md/raid10: refactor some codes from raid10_write_request

Previously, we clone both bio and repl_bio in raid10_write_request,
then add the cloned bio to plug->pending or conf->pending_bio_list
based on plug or not, and most of the logics are same for the two
conditions.

So introduce raid10_write_one_disk for it, and use replacement parameter
to distinguish the difference. No functional changes in the patch.

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