perf symbols: Use correct filename for compressed modules in build-id cache

The decompress_kmodule() decompresses kernel modules in order to load
symbols from it.  In the DSO_BINARY_TYPE__BUILD_ID_CACHE case, it needs
the full file path to extract the file extension to determine the
decompression method.  But overwriting 'name' will fail the
decompression since it might point to a non-existing old file.

Instead, use dso->long_name for having the correct extension and use the
real filename to decompress.

In the DSO_BINARY_TYPE__SYSTEM_PATH_KMODULE_COMP case, both names should
be the same.  This allows resolving symbols in the old modules.

Before:

  $ perf report -i perf.data.old | grep scsi_mod
     0.00%  cc1      [scsi_mod]    [k] 0x0000000000004aa6
     0.00%  as       [scsi_mod]    [k] 0x00000000000099e1
     0.00%  cc1      [scsi_mod]    [k] 0x0000000000009830
     0.00%  cc1      [scsi_mod]    [k] 0x0000000000001b8f

After:

     0.00%  cc1      [scsi_mod]    [k] scsi_handle_queue_ramp_up
     0.00%  as       [scsi_mod]    [k] scsi_sg_alloc
     0.00%  cc1      [scsi_mod]    [k] scsi_setup_cmnd
     0.00%  cc1      [scsi_mod]    [k] scsi_get_command

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: kernel-team@lge.com
Link: http://lkml.kernel.org/r/20170531120105.21731-3-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/symbol-elf.c

@@ -649,10 +649,7 @@ static int decompress_kmodule(struct dso *dso, const char *name,
 	    type != DSO_BINARY_TYPE__BUILD_ID_CACHE)
 		return -1;
 
-	if (type == DSO_BINARY_TYPE__BUILD_ID_CACHE)
-		name = dso->long_name;
-
-	if (kmod_path__parse_ext(&m, name) || !m.comp)
+	if (kmod_path__parse_ext(&m, dso->long_name) || !m.comp)
 		return -1;
 
 	fd = mkstemp(tmpbuf);