md/raid5: Move read_seqcount_begin() into make_stripe_request()

Now that prepare_to_wait() isn't in the way, move read_sequcount_begin()
into make_stripe_request().

No functional changes intended.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>