net: ipv6: ndisc: fix ndisc_ra_useropt to initialize nduseropt_padX fields to zero...

net: ipv6: ndisc: fix ndisc_ra_useropt to initialize nduseropt_padX fields to zero to prevent an info-leak

When processing Router Advertisements with user options the kernel
builds an RTM_NEWNDUSEROPT netlink message. The nduseroptmsg struct
has three padding fields that are never zeroed and can leak kernel data

The fix is simple, just zeroes the padding fields.

Fixes: 31910575a9de ("[IPv6]: Export userland ND options through netlink (RDNSS support)")
Signed-off-by: Yochai Eisenrich <echelonh@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260324224925.2437775-1-echelonh@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>