Upon inspection it looked like the error handling for mls_export_cat() was
rather poor.  This patch addresses this by NULL'ing out kfree()'d pointers
before returning and checking the return value of the function everywhere
it is called.
Signed-off-by: NPaul Moore <paul.moore@hp.com>
Signed-off-by: NJames Morris <jmorris@namei.org>