CVE-2026-52985: netdevsim: zero initialize struct iphdr in dummy sk_buff

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

netdevsim: zero initialize struct iphdr in dummy skbuff

Syzbot reports a KMSAN uninit-value originating from nsimdevtrapskbbuild, with the allocation also being performed in the same function.

Fix this by calling skbputzero instead of skbput to guarantee zero initialization of the whole IP header.