bpftool: Show also the name of type BPF_OBJ_LINK

For example, /sys/fs/bpf/maps.debug is a BPF link. When you run `bpftool map show`
to show it:

Before:

  $ bpftool map show pinned /sys/fs/bpf/maps.debug
  Error: incorrect object type: unknown

After:

  $ bpftool map show pinned /sys/fs/bpf/maps.debug
  Error: incorrect object type: link
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220629154832.56986-5-laoar.shao@gmail.com

tools/bpf/bpftool/common.c

@@ -316,6 +316,7 @@ const char *get_fd_type_name(enum bpf_obj_type type)
 		[BPF_OBJ_UNKNOWN]	= "unknown",
 		[BPF_OBJ_PROG]		= "prog",
 		[BPF_OBJ_MAP]		= "map",
+		[BPF_OBJ_LINK]		= "link",
 	};
 
 	if (type < 0 || type >= ARRAY_SIZE(names) || !names[type])