LU-163 MDS returns 32/64-bit dir name hash according to client type

LU-163 MDS returns 32/64-bit dir name hash according to client type

1) liblustre client, 1.8.5 or older client do not support 64-bit dir name hash.
2) register Lustre version 'get_name()' for NFS processing.
3) 'll_dir_seek()' processes name hash-based dir seek operation.
4) re-define "DIR_END_OFF" as "0x7fffffffffffffffULL" to always return positive
   dir name hash for successful seek.
5) small cleanup for 'll_dops_init()' to drop unnecessary statahead process.

Signed-off-by: nasf <yong.fan@whamcloud.com>
Change-Id: I0e3177c98c6e8814c39551ab54edfa3f861cbc8c
Reviewed-on: http://review.whamcloud.com/432
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>