raid5: fix a small race condition

commit 5f9d1fde(raid5: fix memory leak of bio integrity data)
moves bio_reset to bio_endio. But it introduces a small race condition.
It does bio_reset after raid5_release_stripe, which could make the
stripe reusable and hence reuse the bio just before bio_reset. Moving
bio_reset before raid5_release_stripe is called should fix the race.

Reported-and-tested-by: Stefan Priebe - Profihost AG <s.priebe@profihost.ag>
Signed-off-by: Shaohua Li <shli@fb.com>