CVE-2026-98571: 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 stmmacrx(), when napibuildskb() fails the descriptor page is recycled back to the page pool with pagepoolrecycledirect(), 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 stmmacrxrefill() skips the replacement allocation and programs the already-recycled page back into the RX descriptor.

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