fix: don't make IBPB mandatory when it's not there

On some kernels there could be IBRS support but not
IBPB support, in that case, don't report VULN just
because IBPB is not enabled when IBRS is

spectre-meltdown-checker.sh

@@ -1011,6 +1011,7 @@ check_variant2()
 						ibpb_enabled=$(cat "$dir/ibpb_enabled" 2>/dev/null)
 						_debug "ibpb: found $dir/ibpb_enabled=$ibpb_enabled"
 					else
+						ibpb_enabled=-1
 						_debug "ibpb: no ibpb_enabled file in $dir"
 					fi
 					break
@@ -1193,6 +1194,9 @@ check_variant2()
 		elif [ "$opt_live" = 1 ]; then
 			if [ "$ibrs_enabled" = 1 -o "$ibrs_enabled" = 2 ] && [ "$ibpb_enabled" = 1 ]; then
 				pvulnstatus $cve OK "IBRS/IBPB are mitigating the vulnerability"
+			elif [ "$ibrs_enabled" = 1 -o "$ibrs_enabled" = 2 ] && [ "$ibpb_enabled" = -1 ]; then
+				# IBPB doesn't seem here on this kernel
+				pvulnstatus $cve OK "IBRS is mitigating the vulnerability"
 			elif [ "$ibpb_enabled" = 2 ]; then
 				pvulnstatus $cve OK "Full IBPB is mitigating the vulnerability"
 			else