nfc: llcp: bound SNL TLV parsing to the skb and add length checks

Summary

In the Linux kernel, the following vulnerability has been resolved: nfc: llcp: bound SNL TLV parsing to the skb and add length checks nfc_llcp_recv_snl() walked the SNL TLV list using a u16 offset/length pair derived from skb->len, without bounding reads to the actual skb data. Three problems followed: - For a short frame (skb->len < LLCP_HEADER_SIZE), tlv_len underflowed. - The per-TLV header (type, length) was read without checking that two bytes remained. - A declared TLV length could run past the end of the buffer, and an SDREQ with length == 0 made "service_name_len = length - 1" underflow (size_t), driving an out-of-bounds read in the following strncmp() / nfc_llcp_sock_from_sn(). The SDRES case likewise read tlv[2]/tlv[3] without a length check. A nearby NFC device can reach this without authentication; LLCP link activation happens automatically after NFC-DEP. Walk the TLV list by pointer, bounded by skb_tail_pointer() over the linear skb data, and validate each TLV declared length before use. Add explicit length checks for SDREQ (>= 1) and SDRES (exactly 2). Found by 0sec automated security-research tooling (https://0sec.ai).