IMA: remove read permissions on the ima policy file

The IMA policy file does not implement read.  Trying to just open/read/close
the file will load a blank policy and you cannot then change the policy
without a reboot.  This removes the read permission from the file so one must
at least be attempting to write...

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>