bpf: hide the unused 'off' variable

The local variable is only used while CONFIG_IPV6 enabled

net/core/filter.c: In function ‘sk_msg_convert_ctx_access’:
net/core/filter.c:6489:6: warning: unused variable ‘off’ [-Wunused-variable]
  int off;
      ^
This puts it into #ifdef.

Fixes: 303def35f64e ("bpf: allow sk_msg programs to read sock fields")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -6486,7 +6486,9 @@ static u32 sk_msg_convert_ctx_access(enum bpf_access_type type,
 				     struct bpf_prog *prog, u32 *target_size)
 {
 	struct bpf_insn *insn = insn_buf;
+#if IS_ENABLED(CONFIG_IPV6)
 	int off;
+#endif
 
 	switch (si->off) {
 	case offsetof(struct sk_msg_md, data):