none of thread needs to wait for mount completion in kernel space.

- small cleanups in ll_gns_mount_object(). Some checks for input invariants are
  moved after checking gns state. Fixes in comments.

- added the ability to disable GNS at all via /proc/.../gns_enabled. This is useful
  for some complex tests (like recursive suid marked .mntinfo dir) at least. May be
  useful for something else.

- added two new sanity tests to sanity-gns.sh:

  a) test 2g: mounting /mnt/lustre/gns_entry/.mntinfo/.mntinfo/.mntinfo/.mntinfo where
     only last entry is correct GNS mount object.

  b) test 2h: upcall script starts mounting in background and retuns with no waiting
     for mount command.

- in sanity tests disable GNS when preparing mount objects and mount points.