md: update the optimal I/O size on reshape

[ Upstream commit 16ef510139315a2147ee7525796f8dbd4e4b7864 ]

The raid5 and raid10 drivers currently update the read-ahead size,
but not the optimal I/O size on reshape.  To prepare for deriving the
read-ahead size from the optimal I/O size make sure it is updated
as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: f0ddb83da3cb ("md/raid10: fix memleak of md thread")
Signed-off-by: Sasha Levin <sashal@kernel.org>