tools/bpftool: Emit name <anon> for anonymous BTFs

mainline inclusion
from mainline-5.11-rc1
commit 71ccb500
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=71ccb50074f31a50a1da4c1d8306d54da0907b00

-------------------------------------------------

For consistency of output, emit "name <anon>" for BTFs without the name. This
keeps output more consistent and obvious.
Suggested-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201202065244.530571-2-andrii@kernel.org
(cherry picked from commit 71ccb500)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

tools/bpf/bpftool/btf.c

@@ -754,6 +754,8 @@ show_btf_plain(struct bpf_btf_info *info, int fd,
 		printf("name [%s]  ", name);
 	else if (name && name[0])
 		printf("name %s  ", name);
+	else
+		printf("name <anon>  ");
 	printf("size %uB", info->btf_size);
 
 	n = 0;