perf symbols: Avoid resolving [kernel.kallsyms] to real path for buildid cache

kallsyms has a virtual file name [kernel.kallsyms].  Currently, it can't
be added to buildid cache successfully because the code
(build_id_cache__add_s) tries to resolve [kernel.kallsyms] to a real
absolute pathname and that fails.

Fixes it by not resolving it and just use the name [kernel.kallsyms].
So dir ~/.debug/[kernel.kallsyms] is created.

Original bug report at:
https://lkml.org/lkml/2011/3/1/524

Tested-by: Han Pingtian <phan@redhat.com>
Cc: Han Pingtian <phan@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1299165837-27817-1-git-send-email-ming.m.lin@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -270,11 +270,15 @@ int build_id_cache__add_s(const char *sbuild_id, const char *debugdir,
 			  const char *name, bool is_kallsyms)
 {
 	const size_t size = PATH_MAX;
-	char *realname = realpath(name, NULL),
-	     *filename = malloc(size),
+	char *realname, *filename = malloc(size),
 	     *linkname = malloc(size), *targetname;
 	int len, err = -1;
 
+	if (is_kallsyms)
+		realname = (char *)name;
+	else
+		realname = realpath(name, NULL);
+
 	if (realname == NULL || filename == NULL || linkname == NULL)
 		goto out_free;
 
@@ -306,7 +310,8 @@ int build_id_cache__add_s(const char *sbuild_id, const char *debugdir,
 	if (symlink(targetname, linkname) == 0)
 		err = 0;
 out_free:
-	free(realname);
+	if (!is_kallsyms)
+		free(realname);
 	free(filename);
 	free(linkname);
 	return err;