bpftool: Support dumping BTF VAR's "extern" linkage

Add dumping of "extern" linkage for BTF VAR kind. Also shorten
"global-allocated" to "global" to be in line with FUNC's "global".

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210423181348.1801389-2-andrii@kernel.org

tools/bpf/bpftool/btf.c

@@ -71,7 +71,9 @@ static const char *btf_var_linkage_str(__u32 linkage)
 	case BTF_VAR_STATIC:
 		return "static";
 	case BTF_VAR_GLOBAL_ALLOCATED:
-		return "global-alloc";
+		return "global";
+	case BTF_VAR_GLOBAL_EXTERN:
+		return "extern";
 	default:
 		return "(unknown)";
 	}