an "export" flag indicating that we are trying to NFS export the
filesystem, and the MSDOSFS_LARGEFS flag is set on the filesystem,
then deny the mount update and export request. Otherwise,
let the full mount update proceed normally.
MSDOSFS_LARGES and NFS don't mix because of the way inodes are calculated
for MSDOSFS_LARGEFS.
MFC after: 3 days