md/raid5: Cleanup prototype of raid5_get_active_stripe()

Drop the three bools in the prototype of raid5_get_active_stripe()
and replace them with a flags parameter.

At the same time, drop the distinction with __raid5_get_active_stripe().

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Song Liu <song@kernel.org>