ipmr: prevent info-leak in pmr_cache_report()

ipmr: prevent info-leak in pmr_cache_report()

Yiming Qian reported:

<quote>
 ipmr_cache_report()` allocates a report skb with `alloc_skb(128,
 GFP_ATOMIC)` and appends a `struct igmphdr` using `skb_put()`. In the
 non-`IGMPMSG_WHOLEPKT` path it initializes only:

 - `igmp->type`
 - `igmp->code`

 but does not initialize:

 - `igmp->csum`
 - `igmp->group`

 Later, `igmpmsg_netlink_event()` copies the bytes after `sizeof(struct
 igmpmsg)` into the `IPMRA_CREPORT_PKT` netlink attribute and emits
 `RTM_NEWCACHEREPORT` on `RTNLGRP_IPV4_MROUTE_R`.

 As a result, 6 bytes of stale heap data from the skb head are
 disclosed to userspace.
</quote>

Let's use skb_put_zero() instead of skb_put() to fix this bug.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260430070611.4004529-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>