NetLabel: convert the unlabeled accept flag to use RCU

Currently the NetLabel unlabeled packet accept flag is an atomic type and it
is checked for every non-NetLabel packet which comes into the system but rarely
ever changed.  This patch changes this flag to a normal integer and protects it
with RCU locking.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>