net: ethernet: mtk_wed: fix use-after-free panic in mtk_wed_setup_tc_block_cb()

In the Linux kernel, the following vulnerability has been resolved:

net: ethernet: mtk_wed: fix use-after-free panic in mtk_wed_setup_tc_block_cb()

When there are multiple ap interfaces on one band and with WED on,
turning the interface down will cause a kernel panic on MT798X.

Previously, cb_priv was freed in mtk_wed_setup_tc_block() without
marking NULL,and mtk_wed_setup_tc_block_cb() didn't check the value, too.

Assign NULL after free cb_priv in mtk_wed_setup_tc_block() and check NULL
in mtk_wed_setup_tc_block_cb().

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Unable to handle kernel paging request at virtual address 0072460bca32b4f5
Call trace:
mtk_wed_setup_tc_block_cb+0x4/0x38
0xffffffc0794084bc
tcf_block_playback_offloads+0x70/0x1e8
tcf_block_unbind+0x6c/0xc8
...
---------