net: ethernet: mtk_eth_soc: Fix use-after-free in metadata dst teardown

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

net: ethernet: mtk_eth_soc: Fix use-after-free in metadata dst teardown

mtk_free_dev() calls metadata_dst_free() which frees the metadata_dst

with kfree() immediately, bypassing the RCU grace period.

In the RX path, skb_dst_set_noref() sets a non-refcounted pointer from

the skb to the metadata_dst. This function requires RCU read-side

protection and the dst must remain valid until all RCU readers complete.

Since metadata_dst_free() calls kfree() directly, a use-after-free can

occur if any skb still holds a noref pointer to the dst when the driver

tears it down.

Replace metadata_dst_free() with dst_release() which properly goes

through the refcount path: when the refcount drops to zero, it schedules

the actual free via call_rcu_hurry(), ensuring all RCU readers have

completed before the memory is freed.