EX-2975 lipe: implement approximate LRU in lpcc_purge

EX-2975 lipe: implement approximate LRU in lpcc_purge

LRU is actually FIFO based on the atime of cache file. But we
can only use limited memory to sort the atime of all files.
So we implement an approximate LRU in lpcc_purge.

Change-Id: Ic636d97b08ee6424f54a1088dfe07359df1f8f79
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial
Reviewed-on: https://review.whamcloud.com/43224
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>