CVE-2026-23448: net: usb: cdc_ncm: add ndpoffset to NDP16 nframes bounds check

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

net: usb: cdcncm: add ndpoffset to NDP16 nframes bounds check

cdcncmrxverifyndp16() validates that the NDP header and its DPE entries fit within the skb. The first check correctly accounts for ndpoffset:

if ((ndpoffset + sizeof(struct usbcdcncmndp16)) > skbin->len)

but the second check omits it:

if ((sizeof(struct usbcdcncmndp16) + ret (sizeof(struct usbcdcncmdpe16))) > skbin->len)

This validates the DPE array size against the total skb length as if the NDP were at offset 0, rather than at ndpoffset. When the NDP is placed near the end of the NTB (large wNdpIndex), the DPE entries can extend past the skb data buffer even though the check passes. cdcncmrxfixup() then reads out-of-bounds memory when iterating the DPE array.

Add ndpoffset to the nframes bounds check and use structsizet() to express the NDP-plus-DPE-array size more clearly.