EX-3002 lipe: rename {fast,slow}_pool_free to {fast,slow}_pool_max_used

EX-3002 lipe: rename {fast,slow}_pool_free to {fast,slow}_pool_max_used

Rename o_{fast,slow}_pool_free to o_{fast,slow}_pool_max_used and
adjust logic.

In struct pool_list, rename
  pl_avail to pl_used_kb (and adjust logic)
  pl_total to pl_total_kb.

Replace defaults with equivalent values:
  DEF_FAST_POOL_FREE=70 becomes DEF_FAST_POOL_MAX_USED=30
  DEF_SLOW_POOL_FREE=10 becomes DEF_SLOW_POOL_MAX_USED=90

This change does not add, remove, or rename any command line options.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Test-Parameters: trivial testlist=hot-pools
Change-Id: I5f5551230856eabdaa84972218b5dcb73959c029
Reviewed-on: https://review.whamcloud.com/45377
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46120