LU-13609 mgs: fix config_log buffer handling

LU-13609 mgs: fix config_log buffer handling

Fix buffer handling in mgs_list_logs() to list all MGS config_logs
using multiple ioctl calls when we have a large number of targets.

Fixes: 1d97a8b4cd3d ("LU-13609 llog: list all the log files correctly on MGS/MDT")
Signed-off-by: Stephane Thiell <sthiell@stanford.edu>
Change-Id: I1bf32e918e242f4da83c3d1624b7285a18a88d01
Reviewed-on: https://review.whamcloud.com/41478
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>