LU-11485 lod: disallow setting the last non-stale mirror as stale

LU-11485 lod: disallow setting the last non-stale mirror as stale

"lfs setstripe" allows setting stale flag on the last
non-stale mirror of a file, which makes the file have
no valid component to read and return IO error.

This patch fixes the above issue by disallowing that.
It also disallows "lfs mirror split" to destroy the
last non-stale mirror of a file.

Change-Id: I6934cfe0190cd1ea83de1cf28ddf840b9f96193a
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36141
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>