implementation. This flag will indicate that the security label
in the vnode is currently valid, and therefore doesn't need to
be refreshed before an access control decision can be made. Most
file systems (or stdvops) will set this flag after they load the
MAC label from disk the first time to prevent redundant disk I/O;
some file synthetic file systems (procfs, for example) may not.
Obtained from: TrustedBSD Project
Sponsored by: DARPA, NAI Labs
