netdevsim: zero initialize struct iphdr in dummy sk_buff

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.

Closes: https://syzkaller.appspot.com/bug?extid=23d7fcd204e3837866ff
Fixes: da58f90f11f5 ("netdevsim: Add devlink-trap support")
Signed-off-by: Nikola Z. Ivanov <zlatistiv@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260426201434.742030-1-zlatistiv@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>