net: stmmac: fix kernel panic due to NULL pointer dereference of xsk_pool

After free xsk_pool, there is possibility that napi polling is still
running in the middle, thus causes a kernel crash due to kernel NULL
pointer dereference of rx_q->xsk_pool and tx_q->xsk_pool.

Fix this by changing the XDP pool setup sequence to:
 1. disable napi before free xsk_pool
 2. enable napi after init xsk_pool

The following kernel panic is observed without this patch:

RIP: 0010:xsk_uses_need_wakeup+0x5/0x10
Call Trace:
stmmac_napi_poll_rxtx+0x3a9/0xae0 [stmmac]
__napi_poll+0x27/0x130
net_rx_action+0x233/0x280
__do_softirq+0xe2/0x2b6
run_ksoftirqd+0x1a/0x20
smpboot_thread_fn+0xac/0x140
? sort_range+0x20/0x20
kthread+0x124/0x150
? set_kthread_struct+0x40/0x40
ret_from_fork+0x1f/0x30
---[ end trace a77c8956b79ac107 ]---

Fixes: bba2556e ("net: stmmac: Enable RX via AF_XDP zero-copy")
Cc: <stable@vger.kernel.org> # 5.13.x
Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>