Medium [CVE-2026-74579] fix mask build for partial field offload

In the Linux kernel, the following vulnerability has been resolved: netfilter: nft_payload: fix mask build for partial field offload nft_payload_offload_mask() builds the offload match mask for a payload expression that covers only part of a header field.

For a partial IPv6 address match (field_len = 16, priv_len = 1) that shift is 1 << 120, which is undefined on the 32-bit int operand. It also trims only one word, so the remaining words stay 0xffffffff (and when priv_len is a multiple of 4 the trim is skipped entirely), leaving the mask covering more bytes than the rule matches.

UBSAN: shift-out-of-bounds in net/netfilter/nft_payload.c:278:20 shift exponent 120 is too large for 32-bit type 'int'... The match is byte-granular and struct nft_data is zero-initialised, so the correct mask is simply the first priv_len bytes set to 0xff.

Set those bytes directly and drop the word/shift trimming; this removes the undefined shift and no longer over-masks the trailing bytes. The `nft_payload_offload_mask()` function, which builds offload match masks for network packet header fields, contains a shift-out-of-bounds error.

This occurs when processing partial IPv6 address matches, leading to an incorrect mask calculation and undefined behavior. This vulnerability could potentially result in a denial of service (DoS).