LU-1548 mdssurvey: fix file_count in mds_survey

LU-1548 mdssurvey: fix file_count in mds_survey

It should use file_count_per_thread in lctl command,
instead of file_count directly.

Signed-off-by: Wang di <di.wang@whamcloud.com>
Change-Id: I606a988db373c1065544385fdd1dfd42821eef2b
Reviewed-on: http://review.whamcloud.com/3549
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>