md/raid5: Move common stripe get code into new find_get_stripe() helper

Both uses of find_stripe() require a fairly complicated dance to
increment the reference count. Move this into a common find_get_stripe()
helper.

No functional changes intended.

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