md/raid5: fix a recently broken BUG_ON().

commit 6d183de
    md/raid5: fix newly-broken locking in get_active_stripe.

simplified a BUG_ON, but removed too much so now it sometimes fires
when it shouldn't.

When the STRIPE_EXPANDING flag is set, the stripe_head might be on a
special list while multiple stripe_heads are collected, or it might
not be on any list, even a 'free' list when the refcount is zero.  As
long as STRIPE_EXPANDING is set, it will be found and added back to a
list eventually.

So both of the BUG_ONs which test for the ->lru being empty or not
need to avoid the case where STRIPE_EXPANDING is set.

The patch which broke this was marked for -stable, so this patch needs
to be applied to any branch that received 6d183de

Fixes: 6d183de
Cc: stable@vger.kernel.org (any release to which above was applied)
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -687,7 +687,8 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
 			} else {
 				if (!test_bit(STRIPE_HANDLE, &sh->state))
 					atomic_inc(&conf->active_stripes);
-				BUG_ON(list_empty(&sh->lru));
+				BUG_ON(list_empty(&sh->lru) &&
+				       !test_bit(STRIPE_EXPANDING, &sh->state));
 				list_del_init(&sh->lru);
 				if (sh->group) {
 					sh->group->stripes_cnt--;