ssh_filter_btrbk.sh: whitelist mkdir for 'btrbk archive' operations

doc/ssh_filter_btrbk.1.asciidoc

@@ -56,7 +56,8 @@ OPTIONS
     "btrfs send". Equivalent to "--snapshot --send".
 
 -t, --target::
-    Allow commands for backup target: "btrfs receive".
+    Allow commands for backup and archive target: "btrfs receive",
+    "mkdir".
 
 -d, --delete::
     Allow commands for subvolume deletion: "btrfs subvolume

ssh_filter_btrbk.sh

@@ -125,6 +125,7 @@ while [[ "$#" -ge 1 ]]; do
 
       -t|--target)
           allow_cmd "${sudo_prefix}btrfs receive"
+          allow_cmd "${sudo_prefix}mkdir"
           ;;
 
       -c|--compress)