- fixes and cleanups in GNS:

  - mounting only on lookup() control path (revalidate() for possible mounts always return 0).
    This makes it possible to use lookup_one_len() from ll_gns_mount_object(), as lookup
    control path protected parent.
  - on revalidate cause mount for possible mount points only if revalidate is successful
  - removed not used dentry marking with MOUNT_PENDING, etc