zebra: add error check condition to sock option

zebra: add error check condition to sock option

Adding error checking condition which was missed
in PR-11216.

*** CID 1517953:  Error handling issues  (CHECKED_RETURN)
/zebra/kernel_netlink.c: 313 in netlink_socket()
307                     memset(&snl, 0, sizeof(snl));
308                     snl.nl_family = AF_NETLINK;
309                     snl.nl_groups = groups;
310
311     #if defined SOL_NETLINK
312                     if (ext_groups)
>>>     CID 1517953:  Error handling issues  (CHECKED_RETURN)
>>>     Calling "setsockopt(sock, 270, 1, &ext_groups, 8U)" without checking return value. This library function may fail and return an error code.
313                             setsockopt(sock, SOL_NETLINK, NETLINK_ADD_MEMBERSHIP,
314                                        &ext_groups, sizeof(ext_groups));
315     #endif
316
317                     /* Bind the socket to the netlink structure for anything. */
318                     ret = bind(sock, (struct sockaddr *)&snl, sizeof(snl));

Signed-off-by: Chirag Shah <chirag@nvidia.com>