md/raid5: Don't read ->active_stripes if it's not needed

The atomic_read() is not needed in many cases so only do
the read after the first checks are done.

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