ima: define "fs_unsafe" builtin policy

Permit normally denied access/execute permission for files in policy
on IMA unsupported filesystems.  This patch defines "fs_unsafe", a
builtin policy.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Reviewed-by: Dmitry Kasatkin <dmitry.kasatkin@huawei.com>