bpf: fix unused-var without NETDEVICES

A recent commit added new variables only used if CONFIG_NETDEVICES is
set. A simple fix is to only declare these variables if the same
condition is valid.

Other solutions could be to move the code related to SO_BINDTODEVICE
option from _bpf_setsockopt() function to a dedicated one or only
declare these variables in the related "case" section.

Fixes: 70c5899 ("bpf: Allow SO_BINDTODEVICE opt in bpf_setsockopt")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/core/filter.c

@@ -4340,8 +4340,6 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
 			}
 			break;
 		case SO_BINDTODEVICE:
-			ret = -ENOPROTOOPT;
-#ifdef CONFIG_NETDEVICES
 			optlen = min_t(long, optlen, IFNAMSIZ - 1);
 			strncpy(devname, optval, optlen);
 			devname[optlen] = 0;
@@ -4360,7 +4358,6 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
 				dev_put(dev);
 			}
 			ret = sock_bindtoindex(sk, ifindex, false);
-#endif
 			break;
 		default:
 			ret = -EINVAL;