CVE-2026-98591: bpf: Fix bounds check for skb-backed dynptrs

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

bpf: Fix bounds check for skb-backed dynptrs

The skbpointeriflinear() function checks whether a memory region of length len starting at offset off into the skb is in the linear area, and returns a pointer to the region if so. The check currently subtracts between skbheadlen and offset of the check, and since skbheadlen is unsigned the subtraction can underflow. This causes the bounds check to spuriously pass and generate an arbitrary pointer of the form (skb->data + off).

The only user of this helper is currently skb-backed BPF dynptr code. Returning the wrong pointer leads to the dynptr erroneously being backed with invalid memory.

Ensure the subtraction cannot underflow, and fail the check if it would. Use u64 arithmetic to also prevent overflow when calculating (skbheadlen(skb) - off) since off is unsigned.