net/packet: check TX_RING fragments before memory accounting

tpacket_fill_skb() adds the full non-linear payload length to
skb->truesize and sk_wmem_alloc before checking MAX_SKB_FRAGS. With
PACKET_VNET_HDR, userspace can submit a ring-sized frame even when it
greatly exceeds the device MTU.

A near-INT_MAX frame can saturate sk_wmem_alloc before the fragment
loop rejects it. A subsequent refcount_add(0x40000001) can temporarily
turn REFCOUNT_SATURATED into 1. A concurrent TX completion can then
drop that apparent last reference and free the still-live packet
socket before refcount_warn_saturate() restores the counter.

Check that the remaining payload fits in MAX_SKB_FRAGS pages before
accounting it. Include the initial page offset in the calculation,
and move the existing -EFAULT handling out of the fragment loop.
This preserves the accepted packet sizes and error code while
preventing an invalid frame from injecting an oversized byte charge.

Fixes: 69e3c75f4d54 ("net: TX_RING and packet mmap")
Cc: stable@vger.kernel.org
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Kyle Zeng <kylebot@openai.com>
[edumazet: use net_warn_ratelimited() instead of pr_err(), add a Fixes: tag]
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20261028181450.2221013-1-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>