Expand scope of the MLS policy to include a new entry point available

Expand scope of the MLS policy to include a new entry point available
for enforcement:

  mac_mls_check_system_swapon() - Require that the subject and the
  swapfile target vnode labels dominate one another.  An additional
  check is probably needed here to require that the swapfile target
  has a label of mls/high to prevent information leakage through
  swapfiles.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, Network Associates Laboratories