fix: another attempt to avoid sigpipe on grep (#519)

Take this opportunity to factorize all the greps in /proc/cpuinfo
into a helper that avoids using a pipe to entirely avoid SIGPIPE
on a possibly gigantic /proc/cpuinfo

(cherry picked from commit 5bbffaf053)

src/vulns/CVE-2018-3646.sh

@@ -89,7 +89,7 @@ check_CVE_2018_3646_linux() {
 
         pr_info "* Mitigation 2"
         pr_info_nol "  * L1D flush is supported by kernel: "
-        if [ "$g_mode" = live ] && grep -qw flush_l1d "$g_procfs/cpuinfo"; then
+        if [ "$g_mode" = live ] && cpuinfo_has_flag flush_l1d; then
             l1d_kernel="found flush_l1d in $g_procfs/cpuinfo"
         fi
         if [ -z "$l1d_kernel" ]; then
@@ -162,7 +162,7 @@ check_CVE_2018_3646_linux() {
 
         pr_info_nol "  * Hardware-backed L1D flush supported: "
         if [ "$g_mode" = live ]; then
-            if grep -qw flush_l1d "$g_procfs/cpuinfo" || [ -n "$l1d_xen_hardware" ]; then
+            if cpuinfo_has_flag flush_l1d || [ -n "$l1d_xen_hardware" ]; then
                 pstatus green YES "performance impact of the mitigation will be greatly reduced"
             else
                 pstatus blue NO "flush will be done in software, this is slower"