LU-18803 lmv: add statfs stats to LMV device

LU-18803 lmv: add statfs stats to LMV device

Enable the statfs stats for LMV to show aggregate stats, including
the marged (minimum) os_namelen from all MDTs.

Merge os_state together from targets.  Merge "downgrade" flags if
any target has a problem, mask "upgrade" flags if any target does
not have this improvement.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I07451e65b04e5570a025bd9ff2e72f34a00273c7
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58432
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>