wimax/i2400m: fix bad race condition check in RX path

The i2400m->rx_roq data structure is protected against race conditions
with a reference count (i2400m->rx_roq_refcount); the pointer can be
read-referenced under the i2400m->rx_lock spinlock.

The code in i2400m_rx_edata() wasn't properly following access
protocol, performing an invalid check on i2400m->rx_roq (which is
cleared to NULL when the refcount drops to zero). As such, it was
missing to detect when the data structure is no longer valid and
oopsing with a NULL pointer dereference.

This commit fixes said check by verifying, under the rx_lock spinlock,
that i2400m->rx_roq is non-NULL and then increasing the reference
count before dropping the spinlock.
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>

drivers/net/wimax/i2400m/rx.c

@@ -1033,12 +1033,12 @@ void i2400m_rx_edata(struct i2400m *i2400m, struct sk_buff *skb_rx,
 		ro_sn = (reorder >> I2400M_RO_SN_SHIFT) & I2400M_RO_SN;
 
 		spin_lock_irqsave(&i2400m->rx_lock, flags);
-		roq = &i2400m->rx_roq[ro_cin];
-		if (roq == NULL) {
+		if (i2400m->rx_roq == NULL) {
 			kfree_skb(skb);	/* rx_roq is already destroyed */
 			spin_unlock_irqrestore(&i2400m->rx_lock, flags);
 			goto error;
 		}
+		roq = &i2400m->rx_roq[ro_cin];
 		kref_get(&i2400m->rx_roq_refcount);
 		spin_unlock_irqrestore(&i2400m->rx_lock, flags);