netfilter: nft_socket: fix build with CONFIG_SOCK_CGROUP_DATA=n

In some configurations, the sock_cgroup_ptr() function is not available:

net/netfilter/nft_socket.c: In function 'nft_sock_get_eval_cgroupv2':
net/netfilter/nft_socket.c:47:16: error: implicit declaration of function 'sock_cgroup_ptr'; did you mean 'obj_cgroup_put'? [-Werror=implicit-function-declaration]
   47 |         cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
      |                ^~~~~~~~~~~~~~~
      |                obj_cgroup_put
net/netfilter/nft_socket.c:47:14: error: assignment to 'struct cgroup *' from 'int' makes pointer from integer without a cast [-Werror=int-conversion]
   47 |         cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
      |              ^

Change the caller to match the same #ifdef check, only calling it
when the function is defined.

Fixes: e0bb96db

 ("netfilter: nft_socket: add support for cgroupsv2")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>