veristat: Strerror expects positive number (errno)

Before:

  ./veristat -G @foobar iters.bpf.o
  Failed to open presets in 'foobar': Unknown error -2
  ...

After:

  ./veristat -G @foobar iters.bpf.o
  Failed to open presets in 'foobar': No such file or directory
  ...

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>
Link: https://lore.kernel.org/bpf/20250301000147.1583999-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

tools/testing/selftests/bpf/veristat.c

@@ -660,7 +660,7 @@ static int append_filter_file(const char *path)
 	f = fopen(path, "r");
 	if (!f) {
 		err = -errno;
-		fprintf(stderr, "Failed to open filters in '%s': %s\n", path, strerror(err));
+		fprintf(stderr, "Failed to open filters in '%s': %s\n", path, strerror(-err));
 		return err;
 	}
 
@@ -1422,7 +1422,7 @@ static int append_var_preset_file(const char *filename)
 	f = fopen(filename, "rt");
 	if (!f) {
 		err = -errno;
-		fprintf(stderr, "Failed to open presets in '%s': %s\n", filename, strerror(err));
+		fprintf(stderr, "Failed to open presets in '%s': %s\n", filename, strerror(-err));
 		return -EINVAL;
 	}