md/raid5: Move stripe_request_ctx up

Move stripe_request_ctx up. No functional changes intended.

This will be necessary in the next patch to release the batch_last
in the context before sleeping.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>