EX-3002 lipe: rename lpurge free{lo,hi} to {max,min}-used

EX-3002 lipe: rename lpurge free{lo,hi} to {max,min}-used

Expressing things in terms of used rather than free is much easier to
understand.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Test-Parameters: trivial testlist=hot-pools
Change-Id: I31ff88db97c0dd1b2ddbaa67aa27e8bec520c7d3
Reviewed-on: https://review.whamcloud.com/46903
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandre Ioffe <aioffe@ddn.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>