NetLabel: return the correct error for translated CIPSOv4 tags

The CIPSOv4 translated tag #1 mapping does not always return the correct error
code if the desired mapping does not exist; instead of returning -EPERM it
returns -ENOSPC indicating that the buffer is not large enough to hold the
translated value.  This was caused by failing to check a specific error
condition.  This patch fixes this so that unknown mappings return
-EPERM which is consistent with the rest of the related CIPSOv4 code.

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