IMA: use current_cred() instead of current->cred

Proper invocation of the current credentials is to use current_cred() not
current->cred.  This patches makes IMA use the new method.

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