iscsi-target: fix extract_param to handle buffer length corner case

extract_param() is called with max_length set to the total size of the
output buffer.  It's not safe to allow a parameter length equal to the
buffer size as the terminating null would be written one byte past the
end of the output buffer.
Signed-off-by: Eric Seppanen <eric@purestorage.com>
Cc: <stable@vger.kernel.org> #3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_nego.c

@@ -88,7 +88,7 @@ int extract_param(
 	if (len < 0)
 		return -1;
 
-	if (len > max_length) {
+	if (len >= max_length) {
 		pr_err("Length of input: %d exceeds max_length:"
 			" %d\n", len, max_length);
 		return -1;