dmi_scan: fix missing check for _DMI_ signature in smbios_present()

Commit 9f9c9cbb ("drivers/firmware/dmi_scan.c: fetch dmi version
from SMBIOS if it exists") hoisted the check for "_DMI_" into
dmi_scan_machine(), which means that we don't bother to check for
"_DMI_" at offset 16 in an SMBIOS entry.  smbios_present() may also call
dmi_present() for an address where we found "_SM_", if it failed further
validation.

Check for "_DMI_" in smbios_present() before calling dmi_present().

[akpm@linux-foundation.org: fix build]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Reported-by: Tim McGrath <tmhikaru@gmail.com>
Tested-by: Tim Mcgrath <tmhikaru@gmail.com>
Cc: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/firmware/dmi_scan.c

@@ -442,7 +442,6 @@ static int __init dmi_present(const char __iomem *p)
 static int __init smbios_present(const char __iomem *p)
 {
 	u8 buf[32];
-	int offset = 0;
 
 	memcpy_fromio(buf, p, 32);
 	if ((buf[5] < 32) && dmi_checksum(buf, buf[5])) {
@@ -461,9 +460,9 @@ static int __init smbios_present(const char __iomem *p)
 			dmi_ver = 0x0206;
 			break;
 		}
-		offset = 16;
+		return memcmp(p + 16, "_DMI_", 5) || dmi_present(p + 16);
 	}
-	return dmi_present(buf + offset);
+	return 1;
 }
 
 void __init dmi_scan_machine(void)