EX-3301 lipe: use larger candidate number in lpcc_purge

EX-3301 lipe: use larger candidate number in lpcc_purge

Increase the default candidate number to 128K. Calculate the
number of n_discard and n_detach dynamically based on the
candidate number. Set timeout of continious scanning.

Change-Id: I8adacb722fdec820a914250c54d05e0abd740140
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial
Reviewed-on: https://review.whamcloud.com/44040
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>