net: stmmac: clear stale buf->page after recycling on skb build failure

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

net: stmmac: clear stale buf->page after recycling on skb build failure

In stmmac_rx(), when napi_build_skb() fails the descriptor page is
recycled back to the page pool with page_pool_recycle_direct(), but
buf->page is left pointing at the recycled page, unlike every other
consumption site in the function which clears the pointer after handing
the page away.

With the stale pointer stmmac_rx_refill() skips the replacement
allocation and programs the already-recycled page back into the RX
descriptor.

Clear buf->page on the napi_build_skb() failure path to keep the buffer
lifecycle consistent with the other consumption sites.