netlink: correct nlmsg size for multicast notifications

Corrected the netlink message size calculation for multicast group
join/leave notifications. The previous calculation did not account for
the inclusion of both IPv4/IPv6 addresses and ifa_cacheinfo in the
payload. This fix ensures that the allocated message size is
sufficient to hold all necessary information.

This patch also includes the following improvements:
* Uses GFP_KERNEL instead of GFP_ATOMIC when holding the RTNL mutex.
* Uses nla_total_size(sizeof(struct in6_addr)) instead of
  nla_total_size(16).
* Removes unnecessary EXPORT_SYMBOL().

Fixes: 2c2b61d2 ("netlink: add IGMP/MLD join/leave notifications")
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Link: https://patch.msgid.link/20241221100007.1910089-1-yuyanghuang@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>