RDMA/rdmavt: Catch use-after-free access of AH structures

Prior to commit d345691 ("RDMA: Handle AH allocations by IB/core"),
AH destroy path is rdmavt returned -EBUSY warning to application and
caused to potential leakage of kernel memory of AH structure.

After that commit, the AH structure is always freed but such early return
in driver code can potentially cause to use-after-free error.

Add warning to catch such situation to help driver developers to fix AH
release path.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/sw/rdmavt/ah.c

@@ -141,8 +141,7 @@ void rvt_destroy_ah(struct ib_ah *ibah, u32 destroy_flags)
 	struct rvt_ah *ah = ibah_to_rvtah(ibah);
 	unsigned long flags;
 
-	if (atomic_read(&ah->refcount) != 0)
-		return;
+	WARN_ON_ONCE(atomic_read(&ah->refcount));
 
 	spin_lock_irqsave(&dev->n_ahs_lock, flags);
 	dev->n_ahs_allocated--;