audit: fix uninitialized variable in audit_add_rule()

As reported by the 0-Day testing service:

   kernel/auditfilter.c: In function 'audit_rule_change':
>> kernel/auditfilter.c:864:6: warning: 'err' may be used uninit...
     int err;

Cc: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <pmoore@redhat.com>