zloop.sh requires bash

The zloop.sh script requires bash.  It will require further improvements
to be compatible with the alternatives such as dash.  This resolves the
ztest failures observed under Ubuntu in the automated tested.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4441

scripts/zloop.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 #
 # CDDL HEADER START