perf diff: Do not overwrite valid build id

Fixes a perf diff regression issue which was introduced by commit
5baecbcd9c9a ("perf symbols: we can now read separate debug-info files
based on a build ID")

The binary name could be same when perf diff different binaries. Build
id is used to distinguish between them.
However, the previous patch assumes the same binary name has same build
id. So it overwrites the build id according to the binary name,
regardless of whether the build id is set or not.

Check the has_build_id in dso__load. If the build id is already set, use
it.

Before the fix:

$ perf diff 1.perf.data 2.perf.data
# Event 'cycles'
#
# Baseline Delta Shared Object Symbol
# ........ ....... ................ .............................
#
99.83% -99.80% tchain_edit [.] f2
0.12% +99.81% tchain_edit [.] f3
0.02% -0.01% [ixgbe] [k] ixgbe_read_reg

After the fix:
$ perf diff 1.perf.data 2.perf.data
# Event 'cycles'
#
# Baseline Delta Shared Object Symbol
# ........ ....... ................ .............................
#
99.83% +0.10% tchain_edit [.] f3
0.12% -0.08% tchain_edit [.] f2

Signed-off-by: Kan Liang <[e-mail]>
Cc: Andi Kleen <[e-mail]>
CC: Dima Kogan <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Fixes: 5baecbcd9c9a ("perf symbols: we can now read separate debug-info files based on a build ID")
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/symbol.c

···
 	 * Read the build id if possible. This is required for
 	 * DSO_BINARY_TYPE__BUILDID_DEBUGINFO to work
 	 */
-	if (is_regular_file(dso->long_name) &&
+	if (!dso->has_build_id &&
+	    is_regular_file(dso->long_name) &&
 	    filename__read_build_id(dso->long_name, build_id, BUILD_ID_SIZE) > 0)
 		dso__set_build_id(dso, build_id);