UBUNTU: SAUCE: s390: Add 'nogmb' kernel parameter

BugLink: http://bugs.launchpad.net/bugs/1768474

The update of the SAUCE s390 Spectre patch set to the upstream version
from 4.4.130 removed the 'nogmb' kernel parameter. Add it back to retain
the original behavior.
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/s390/kernel/alternative.c

@@ -15,6 +15,13 @@ static int __init disable_alternative_instructions(char *str)
 
 early_param("noaltinstr", disable_alternative_instructions);
 
+static int __init nogmb_setup_early(char *str)
+{
+	__clear_facility(81, S390_lowcore.alt_stfle_fac_list);
+	return 0;
+}
+early_param("nogmb", nogmb_setup_early);
+
 struct brcl_insn {
 	u16 opc;
 	s32 disp;