IMA: set entry->action to UNKNOWN rather than hard coding

ima_parse_rule currently sets entry->action = -1 and then later tests
if (entry->action == UNKNOWN).  It is true that UNKNOWN == -1 but actually
setting it to UNKNOWN makes a lot more sense in case things change in the
future.

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