wmi: use memcmp instead of strncmp to compare GUIDs

While looking for the duplicates in /sys/class/wmi/, I couldn't find
them. The code that looks for duplicates uses strncmp in a binary GUID,
which may contain zero bytes. The right function is memcmp, which is
also used in another section of wmi code.

It was finding 49142400-C6A3-40FA-BADB-8A2652834100 as a duplicate of
39142400-C6A3-40FA-BADB-8A2652834100. Since the first byte is the fourth
printed, they were found as equal by strncmp.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: stable@kernel.org

drivers/platform/x86/wmi.c

@@ -755,7 +755,7 @@ static bool guid_already_parsed(const char *guid_string)
 	struct wmi_block *wblock;
 
 	list_for_each_entry(wblock, &wmi_block_list, list)
-		if (strncmp(wblock->gblock.guid, guid_string, 16) == 0)
+		if (memcmp(wblock->gblock.guid, guid_string, 16) == 0)
 			return true;
 
 	return false;