Fix 0011263: Incorrect variable naming in return_dynamic_filters

Commit ce2c32727936bb65405878c4f0ae3967cd340500 refactored
return_dynamic_filters.php

There was an error in that refactoring where a variable name wasn't
properly renamed ($filter to $t_filter_target).

Additionally, this commit renames $t_filter_target to $f_filter_target
in accordance with the coding standards. $f_filter_target is form data
supplied by the user.