LU-13693 lfs: avoid opening regular files for getstripe

LU-13693 lfs: avoid opening regular files for getstripe

In get_mds_md_size() just return a large enough size for all striping
attributes. This saves lfs getstripe from opening the file which was
interfering with leases used for mirroring.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Ia45eb8f6aa942507a55965afccbc28375788dff2
Reviewed-on: https://review.whamcloud.com/38979
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>