In vfs_export(), if we specify MNT_DELEXPORT in the struct export_args,

In vfs_export(), if we specify MNT_DELEXPORT in the struct export_args,
after we perform the operations to delete the export,
call vfs_deleteopt() to delete the "export" mount option from
the linked list of mount options associated with that mount point.

This fixes one scenario:
- put a filesystem in /etc/exports to export it
- remove the filesystem from /etc/exports to delete the export and restart
  mountd
- try to do a "mount -u -o ro" or "mount -u -o rw" on that filesystem
  now that it is no  longer exported.