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 sk_buff Syzbot reports a KMSAN uninit-value originating from nsim_dev_trap_skb_build, with the allocation also being performed in the same function. Fix this by calling skb_put_zero instead of skb_put to guarantee zero initialization of the whole IP header.