SELinux: Only return netlink error when we know the return is fatal

Some of the SELinux netlink code returns a fatal error when the error might
actually be transient.  This patch just silently drops packets on
potentially transient errors but continues to return a permanant error
indicator when the denial was because of policy.

Based-on-comments-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Reviewed-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>