bpftool: Migrate -1 err checks of libbpf fn calls

Per [0], callers of libbpf functions with LIBBPF_STRICT_DIRECT_ERRS set
should handle negative error codes of various values (e.g. -EINVAL).
Migrate two callsites which were explicitly checking for -1 only to
handle the new scheme.

[0]: https://github.com/libbpf/libbpf/wiki/Libbpf-1.0-migration-guide#direct-error-code-returning-libbpf_strict_direct_errs

Signed-off-by: Dave Marchevsky <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/btf_dumper.c

···
 
 	/* Get the bpf_prog's name.  Obtain from func_info. */
 	prog_fd = bpf_prog_get_fd_by_id(prog_id);
-	if (prog_fd == -1)
+	if (prog_fd < 0)
 		goto print;
 
 	prog_info = bpf_program__get_prog_info_linear(prog_fd,

tools/bpf/bpftool/struct_ops.c

···
 	}
 
 	fd = bpf_map_get_fd_by_id(id);
-	if (fd == -1) {
+	if (fd < 0) {
 		p_err("can't get map by id (%lu): %s", id, strerror(errno));
 		res.nr_errs++;
 		return res;