netfilter: nft_socket: fix an unused variable warning

The variable is only used in an #ifdef, causing a harmless warning:

net/netfilter/nft_socket.c: In function 'nft_socket_init':
net/netfilter/nft_socket.c:137:27: error: unused variable 'level' [-Werror=unused-variable]
  137 |         unsigned int len, level;
      |                           ^~~~~

Move it into the same #ifdef block.

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>