staging: lustre: llite: move dir cache to MDC layer

Move directory entries cache from llite to MDC, so client
side dir stripe will use independent hash function(in LMV),
which does not need to be tightly coupled with the backend
storage dir-entry hash function. With striped directory, it
will be 2-tier hash, LMV calculate hash value according to the
name and hash-type in layout, then each MDT will store these
entry in disk by its own hash.

Signed-off-by: wang di <di.wang@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3531
Reviewed-on: http://review.whamcloud.com/7043


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>