Add wait_freeing helper function to ZTS

Sometimes the ZTS checks freed space just after `zfs destroy snapshot` and
gets an unexpected value because of space being freed asynchronously.
For cases like this add a `wait_freeing` function which blocks until the
pools `freeing` property drops to zero.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #5740

tests/zfs-tests/tests/functional/cli_root/zfs_set/zfs_set_common.kshlib

@@ -92,6 +92,8 @@ function set_n_check_prop
 		log_mustnot $ZFS set $prop=$expect_value $dataset
 
 		[[ -n $prop ]] && cur_value=$(get_prop $prop $dataset)
+		
+		wait_freeing
 
 		if [[ "$expect_value" != "" && "$cur_value" != "$old_value" ]];
 		then

tests/zfs-tests/tests/functional/snapshot/snapshot_008_pos.ksh

@@ -88,6 +88,8 @@ while [[ $i -lt $COUNT ]]; do
 	(( i = i + 1 ))
 done
 
+wait_freeing $TESTPOOL
+
 new_size=`get_prop available $TESTPOOL`
 
 typeset -i tolerance=0