LU-6138 lfsck: set async windows size properly

LU-6138 lfsck: set async windows size properly

If the async windows size is set as zero, then the LFSCK main engine
on the MDT will pre-load objects as fast as possible. Under such case,
if the peer server(s) cannot handle the pre-load requests in time, it
will cause a lot of pre-load requests waiting on the MDT as to memory
pressure. To avoid such trouble, we will forbid to set the LFSCK async
windows size as zero or other too large (> LFSCK_ASYNC_WIN_MAX) valid.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I3468236a4a0705ea60b49704583b051c99c77cd5
Reviewed-on: http://review.whamcloud.com/13818
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>