tools, bpftool: Print correct error message when failing to load BTF

btf__parse_raw and btf__parse_elf return negative error numbers wrapped
in an ERR_PTR, so the extracted value needs to be negated before passing
them to strerror which expects a positive error number.

Before:
  Error: failed to load BTF from .../vmlinux: Unknown error -2

After:
  Error: failed to load BTF from .../vmlinux: No such file or directory
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200525135421.4154-1-tklauser@distanz.chSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/btf.c

@@ -553,7 +553,7 @@ static int do_dump(int argc, char **argv)
 			btf = btf__parse_elf(*argv, NULL);
 
 		if (IS_ERR(btf)) {
-			err = PTR_ERR(btf);
+			err = -PTR_ERR(btf);
 			btf = NULL;
 			p_err("failed to load BTF from %s: %s",
 			      *argv, strerror(err));