NetLabel: convert a BUG_ON in the CIPSO code to a runtime check

This patch changes a BUG_ON in the CIPSO code to a runtime check.  It should
also increase the readability of the code as it replaces an unexplained
constant with a well defined macro.

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