md/raid5: Factor out ahead_of_reshape() function

There are a few uses of an ugly ternary operator in raid5_make_request()
to check if a sector is a head of a reshape sector.

Factor this out into a simple helper called ahead_of_reshape().

No functional changes intended.

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