CVE-2026-74492: netfilter: ipset: do not update comments from kernel-side hash adds

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

netfilter: ipset: do not update comments from kernel-side hash adds

mtyperesize() copies comment pointers with memcpy(), not the comment objects themselves. During the window after an entry has been copied but before the table swap and backlog replay, the old table is still published for packet-side updates while the replacement-table entry already holds the same ipsetcommentrcu pointer.

If xtSET --add-set ... --exist hits that old entry in this window, mtypeadd() calls ipsetinitcomment() even though packet-side adds carry no comment payload. That call frees the shared comment through the old entry, so the replacement-table entry now holds a stale pointer. When the queued add is replayed on the new table, mtypeadd() calls ipsetinitcomment() again and strlen() dereferences the stale pointer.

Fix this in mtypeadd() by skipping ipsetinitcomment() when ext->target marks a packet-side add. Userspace adds still update comments, while packet-side adds can no longer free comment storage shared with a resize copy.