feat(brctl): for delif commnd offer only bridged interfaces

When completing a delif command offer only interfaces that are part of
the selected bridge.

completions/brctl

@@ -1,4 +1,9 @@
 # bash completion for brctl                                -*- shell-script -*-
+_comp_xfunc_brctl_interfaces()
+{
+    _comp_compgen_split -- "$(${1:-brctl} show ${2:+"$2"} 2>/dev/null | _comp_awk \
+        '(NR == 1) { next }; (/^\t/) { print $1; next }; { print $4 }')"
+}
 
 _comp_cmd_brctl()
 {
@@ -25,9 +30,12 @@ _comp_cmd_brctl()
             ;;
         3)
             case $command in
-                addif | delif)
+                addif)
                     _comp_compgen_available_interfaces
                     ;;
+                delif)
+                    _comp_xfunc_brctl_interfaces "$1" "$prev"
+                    ;;
                 stp)
                     _comp_compgen -- -W 'on off'
                     ;;