rtnetlink: count IFLA_INFO_SLAVE_KIND in if_nlmsg_size

rtnl_link_get_slave_info_data_size counts IFLA_INFO_SLAVE_DATA, but
rtnl_link_slave_info_fill adds both IFLA_INFO_SLAVE_DATA and
IFLA_INFO_SLAVE_KIND.

Fixes: ba7d49b1f0f8 ("rtnetlink: provide api for getting and setting slave info")
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/049843b532e23cde7ddba263c0bbe35ba6f0d26d.1773919462.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>