net: atlantic: fix double free in ring reinit logic

Driver has a logic leak in ring data allocation/free,
where double free may happen in aq_ring_free if system is under
stress and driver init/deinit is happening.

The probability is higher to get this during suspend/resume cycle.

Verification was done simulating same conditions with

    stress -m 2000 --vm-bytes 20M --vm-hang 10 --backoff 1000
    while true; do sudo ifconfig enp1s0 down; sudo ifconfig enp1s0 up; done

Fixed by explicitly clearing pointers to NULL on deallocation

Fixes: 018423e9 ("net: ethernet: aquantia: Add ring support code")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/netdev/CAHk-=wiZZi7FcvqVSUirHBjx0bBUZ4dFrMDVLc3+3HCrtq0rBA@mail.gmail.com/Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Link: https://lore.kernel.org/r/20231213094044.22988-1-irusskikh@marvell.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/aquantia/atlantic/aq_ring.c

@@ -938,11 +938,14 @@ void aq_ring_free(struct aq_ring_s *self)
 		return;
 
 	kfree(self->buff_ring);
+	self->buff_ring = NULL;
 
-	if (self->dx_ring)
+	if (self->dx_ring) {
 		dma_free_coherent(aq_nic_get_dev(self->aq_nic),
 				  self->size * self->dx_size, self->dx_ring,
 				  self->dx_ring_pa);
+		self->dx_ring = NULL;
+	}
 }
 
 unsigned int aq_ring_fill_stats_data(struct aq_ring_s *self, u64 *data)