Fix issue when destroying snapshot of top-level dataset

To extract the pool name from a snapshot name, a parameter expansion was
removing everything after '/'.  However, the top-level dataset contains no '/'
in its name (its name is the same as the pool's name).  This means the entire
snapshot name was being incorrectly returned for the name of the pool.

This commit fixes the parameter expansion to handle the case when the dataset
name contains no '/'.

zap

@@ -200,7 +200,7 @@ destroy () {
     [ -n "$v_opt" ] && printf "%s\nDestroying snapshots...\n" "$(date)"
     for i in $(zfs list -H -t snap -o name); do
         if echo "$i" | grep -E -q "$zptn"; then
-            pool="${i%%/*}"
+            pool="${i%%[/@]*}"
             # Do not quote $D_opt, but ensure it does not contain spaces.
             if ! pool_ok $D_opt "$pool"; then
                 warn "Did not destroy $i because of pool state."