SELinux: restore proper NetLabel caching behavior

A small fix to the SELinux/NetLabel glue code to ensure that the NetLabel
cache is utilized when possible.  This was broken when the SELinux/NetLabel
glue code was reorganized in the last kernel release.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>