Force ztest to always use /dev/urandom

For ztest, which is solely for testing, using a pseudo random
is entirely reasonable.  Using /dev/urandom ensures the system
entropy pool doesn't get depleted thus stalling the testing.
This is a particular problem when testing in VMs.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed by: Thomas Caputi <tcaputi@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7017 
Closes #7036

scripts/zloop.sh

@@ -148,15 +148,6 @@ function store_core
 	fi
 }
 
-rngdpid=""
-function on_exit
-{
-	if [ -n "$rngdpid" ]; then
-		kill -9 "$rngdpid"
-	fi
-}
-trap on_exit EXIT
-
 # parse arguments
 # expected format: zloop [-t timeout] [-c coredir] [-- extra ztest args]
 coredir=$DEFAULTCOREDIR
@@ -205,9 +196,6 @@ or_die rm -f ztest.history
 or_die rm -f ztest.ddt
 or_die rm -f ztest.cores
 
-# start rngd in the background so we don't run out of entropy
-or_die read -r rngdpid < <(rngd -f -r /dev/urandom & echo $!)
-
 ztrc=0		# ztest return value
 foundcrashes=0	# number of crashes found so far
 starttime=$(date +%s)