[PATCH] fix AUDIT_FILTER_PREPEND handling

Clear AUDIT_FILTER_PREPEND flag after adding rule to list.  This
fixes three problems when a rule is added with the -A syntax:

    - auditctl displays filter list as "(null)"
    - the rule cannot be removed using -d
    - a duplicate rule can be added with -a

Signed-off-by: Amy Griffis <amy.griffis@hp.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>