perf dso: Set error code when open() fails on uncompressed fallback path

filename__decompress() has an early return for files that are not
actually compressed, where it calls open() directly.  When open()
fails, the function returns -1 but never sets *err.  The caller chain
(decompress_kmodule → dso__decompress_kmodule_path → dso__get_filename)
then reads *dso__load_errno(dso) to set errno, but that field was never
populated, so errno gets a stale or zero value.

With errno=0, __open_dso() computes fd = -errno = 0, which is non-
negative, so callers treat fd 0 (stdin) as a valid DSO file descriptor.

Set *err = errno when open() fails on the uncompressed path, matching
the error handling on the compressed path at line 354.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Fixes: 8b42b7e5e8 ("perf tools: Add is_compressed callback to compressions array")
Cc: Jiri Olsa <jolsa@kernel.org>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/dso.c

@@ -344,9 +344,12 @@ int filename__decompress(const char *name, char *pathname,
 	 * descriptor to the uncompressed file.
 	 */
 	if (!compressions[comp].is_compressed(name)) {
+		fd = open(name, O_RDONLY | O_CLOEXEC);
+		if (fd < 0)
+			*err = errno;
 		if (pathname && len > 0)
 			pathname[0] = '\0';
-		return open(name, O_RDONLY | O_CLOEXEC);
+		return fd;
 	}
 
 	fd = mkostemp(tmpbuf, O_CLOEXEC);