LU-1254 iokit: should not use single '[]'

Single [] checking for empty variable is dangerous.
When the variable is undefined, it will fall through
without double quote

Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Change-Id: Iec49cd704be8d106cce07032dc20f060f6044e8a
Reviewed-on: http://review.whamcloud.com/2373


Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>

lustre-iokit/obdfilter-survey/libecho

@@ -217,7 +217,7 @@ cleanup () {
     shift
     for ((i = 0; i < $ndevs; i++)); do
         host=${host_names[$i]}
-        if [ -n ${do_teardown_ec[$i]} ]; then
+        if [[ -n "${do_teardown_ec[$i]}" ]]; then
             teardown_ec_devno $host ${client_names[$i]}
         fi
     done