LU-50 kernel: configurable BH LRU size

LU-50 kernel: configurable BH LRU size

This patch allow per-CPU LRU size to be selected at compile time.
More complex filesystems may be modiyfing multiple blocks
within a single transaction, so keeping more buffer heads in
CPU-local cache speeds up modifications significantly.
User can set LRU size to any value between 8 and 64, linux
kernel is still using 8 and Lustre is going using 16 as
default value.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I4bb2eea30d57298b706354dd8438a10dc8ae6e98
Reviewed-on: http://review.whamcloud.com/1796
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>