selinux: ensure that the cached NetLabel secattr matches the desired SID

In selinux_netlbl_skbuff_setsid() we leverage a cached NetLabel
secattr whenever possible.  However, we never check to ensure that
the desired SID matches the cached NetLabel secattr.  This patch
checks the SID against the secattr before use and only uses the
cached secattr when the SID values match.

Signed-off-by: Paul Moore <pmoore@redhat.com>