bpftool: Fix readlink usage in get_fd_type

The `readlink(path, buf, sizeof(buf))` call reads at most sizeof(buf)
bytes and *does not* append null-terminator to buf. With respect to
that, fix two pieces in get_fd_type:

1. Change the truncation check to contain sizeof(buf) rather than
   sizeof(path).
2. Append null-terminator to buf.

Reported by Coverity.

Signed-off-by: Viktor Malik <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Reviewed-by: Quentin Monnet <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

tools/bpf/bpftool/common.c

@@ -461,10 +461,11 @@ int get_fd_type(int fd)
 		p_err("can't read link type: %s", strerror(errno));
 		return -1;
 	}
-	if (n == sizeof(path)) {
+	if (n == sizeof(buf)) {
 		p_err("can't read link type: path too long!");
 		return -1;
 	}
+	buf[n] = '\0';
 
 	if (strstr(buf, "bpf-map"))
 		return BPF_OBJ_MAP;