LU-3066 llite: fix crash in fdatasync(directory)

kernel NULL pointer dereference for fdatasync(directory)

fd = open("/mnt/lustre", O_RDONLY|O_NONBLOCK|O_DIRECTORY);
fdatasync(fd);
close(fd);

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: Ib14b25d1694131e1a65373654008b7f337ce959e
Reviewed-on: http://review.whamcloud.com/5886
Tested-by: Hudson
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>