Refactor sync_request_write() of md/raid10 to use bio_copy_data()
instead of open coding bio_vec iterations.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Neil Brown <neilb@suse.de>
Cc: linux-raid@vger.kernel.org
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Acked-by: NNeilBrown <neilb@suse.de>
Signed-off-by: NKent Overstreet <kent.overstreet@gmail.com>
[dpark: add more description in commit message]
Signed-off-by: NDongsu Park <dpark@posteo.net>
Signed-off-by: NMing Lin <mlin@kernel.org>
Signed-off-by: NNeilBrown <neilb@suse.de>