md/raid5: Refactor raid5_make_request loop

Break immediately if raid5_get_active_stripe() returns NULL and deindent
the rest of the loop. Annotate this check with an unlikely().

This makes the code easier to read and reduces the indentation level.

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>