[NetLabel]: Verify sensitivity level has a valid CIPSO mapping

[NetLabel]: Verify sensitivity level has a valid CIPSO mapping

The current CIPSO engine has a problem where it does not verify that
the given sensitivity level has a valid CIPSO mapping when the "std"
CIPSO DOI type is used.  The end result is that bad packets are sent
on the wire which should have never been sent in the first place.
This patch corrects this problem by verifying the sensitivity level
mapping similar to what is done with the category mapping.  This patch
also changes the returned error code in this case to -EPERM to better
match what the category mapping verification code returns.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: David S. Miller <davem@davemloft.net>