Don't audit spurious dir:write failures

Sepolicy audit is generated for system_file dir when netd executes
iptables-restore command. In this case, O_CREAT flag is passed and
the denial message happens to be a false alarm. Add dontaudit to
remove the denial spam.

CRs-Fixed: 2280747
Change-Id: I7f2927d367815f6d0a04031fdef42531d2743540