The issue has to do with how flags are defined in mount.h. Specifically there are the flags that are externally visible (prefixed with MNT_) and those that are for internal use (prefixed with MNTK_, the K standing for KERNEL). If it is desirable to have MNTK_NULL_NOCACHE visible, then it should be renamed to MNT_NULL_CACHE, added to MNT_VISFLAGMASK, and listed in MNTOPT_NAMES. It probably belongs in the set described as `Flags set by internal operations, but visible to the user.' With this change, it will be displayed by the mount command and show up in the statfs flags.
Kirk McKusick