LU-2139 osc: Move cl_client_lru to cl_client_cache

LU-2139 osc: Move cl_client_lru to cl_client_cache

The cl_client_lru structure was renamed to cl_client_cache. This is
intended as a purely cosmetic change motivated by the future inclusion
of other, non LRU, information in that structure. For example, the plan
is to include "unstable" page information in this structure; which is
unrelated to the LRU list of clean pages, but still part of a client's
cache.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: I286a1bbb2fb0d7dc52f007407bb823f960ae7c62
Reviewed-on: http://review.whamcloud.com/4483
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>