dist: scylla_lib.sh: get_mode_cpu_set: don't let the error messages out

References #3508

Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>
Message-Id: <1529702903-24909-2-git-send-email-vladz@scylladb.com>
(cherry picked from commit f3ca17b1a1)

dist/common/scripts/scylla_lib.sh

@@ -51,7 +51,7 @@ verify_args() {
 #
 get_mode_cpu_set() {
     local mode=$1
-    local mode_cpu_mask=`/usr/lib/scylla/perftune.py --tune net --nic "$nic" --mode "$mode" --get-cpu-mask` 2>&-
+    local mode_cpu_mask=`/usr/lib/scylla/perftune.py --tune net --nic "$nic" --mode "$mode" --get-cpu-mask 2>&-`
 
     # If the given mode is not supported - return invalid CPU set
     if [[ "$?" -ne "0" ]]; then