multipath: work around false shellcheck error

Shellcheck seems to think that [ -z "$debug" ] is setting the debug
variable and warns:

  common/multipath-over-rdma:190:11: note: Modification of debug is local (to subshell caused by pipeline). [SC2030]
  common/multipath-over-rdma:606:8: note: debug was modified in a subshell. That change might be lost. [SC2031]

Work around this by using test instead.

Signed-off-by: Omar Sandoval <[email protected]>

Author: osandov

common/multipath-over-rdma

@@ -187,7 +187,7 @@ remove_stale_mpath_devs() {
 			echo -n "removing $mpdev: "
 			if ! remove_mpath_dev "$mpdev"; then
 				echo "failed"
-				[ -z "$debug" ] || return 1
+				test -z "$debug" || return 1
 			fi
 		done
 	echo "Finished examining multipaths"