efi: cper: fix scnprintf() use in cper_mem_err_location()

The last two if-clauses fail to update n, so whatever they might have
written at &msg[n] would be cut off by the final nul-termination.

That nul-termination is redundant; scnprintf(), just like snprintf(),
guarantees a nul-terminated output buffer, provided the buffer size is
positive.

And there's no need to discount one byte from the initial buffer;
vsnprintf() expects to be given the full buffer size - it's not going
to write the nul-terminator one beyond the given (buffer, size) pair.

Signed-off-by: Rasmus Villemoes <[e-mail]>
Signed-off-by: Ard Biesheuvel <[e-mail]>

drivers/firmware/efi/cper.c

···
 		return 0;
 
 	n = 0;
-	len = CPER_REC_LEN - 1;
+	len = CPER_REC_LEN;
 	if (mem->validation_bits & CPER_MEM_VALID_NODE)
 		n += scnprintf(msg + n, len - n, "node: %d ", mem->node);
 	if (mem->validation_bits & CPER_MEM_VALID_CARD)
···
 		n += scnprintf(msg + n, len - n, "responder_id: 0x%016llx ",
 			       mem->responder_id);
 	if (mem->validation_bits & CPER_MEM_VALID_TARGET_ID)
-		scnprintf(msg + n, len - n, "target_id: 0x%016llx ",
-			  mem->target_id);
+		n += scnprintf(msg + n, len - n, "target_id: 0x%016llx ",
+			       mem->target_id);
 	if (mem->validation_bits & CPER_MEM_VALID_CHIP_ID)
-		scnprintf(msg + n, len - n, "chip_id: %d ",
-			  mem->extended >> CPER_MEM_CHIP_ID_SHIFT);
+		n += scnprintf(msg + n, len - n, "chip_id: %d ",
+			       mem->extended >> CPER_MEM_CHIP_ID_SHIFT);
 
-	msg[n] = '\0';
 	return n;
 }