LU-17481 mdt: count all opens in mdt.*.md_stats

LU-17481 mdt: count all opens in mdt.*.md_stats

Count all of opens for mdt. Also add a test case to
verify it.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I2fa90cc2b4ce8d7d039736a5f40a70cbeb04bf8c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53880
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>