fix folder_snapshot to handle @ snapshots

modules/filesystem/btrfs/btrfs.mod

@@ -112,7 +112,11 @@ folder_snapshot() {
     if [[ -d $2/$sv ]]; then
       echo snapshot $sv already exists in $2. You must manually delete target snapshots!
     else
-      sudo $BTRFS_BIN sub snap $readonly $1/$sv $2
+      if sudo mkdir -p "$2" > /dev/null; then
+      sudo $BTRFS_BIN sub snap $readonly "$1/$sv" "$2/$sv"
+      else
+      echo unable to make directory $2 so can not make subvolumes therein
+      fi
     fi
   done