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.