LU-16026 llite: always enable remote subdir mount

LU-16026 llite: always enable remote subdir mount

For historical reason, ROOT is revalidated with IT_LOOKUP in
.permission to ensure permission is update to date because ROOT is
never looked up. But ROOT FID and layout is not changeable, it's
PERM lock that should be revalidated, i.e., revalidate with
IT_GETATTR instead of IT_LOOKUP.

Since PERM|UPDATE lock is on the MDT where object is located, client
can cache this lock, therefore remote subdir mount doesn't need to
lookup ROOT in each file access.

Deprecate mdt.*.enable_remote_subdir_mount.

Per http://review.whamcloud.com/19195, replace 'df' with 'lfs df' in
sanity 228b since the former doesn't support transparent recovery.

Add sanity 247h.

Lustre-change: https://review.whamcloud.com/48535
Lustre-commit: 6f490275b0e0455a431707775d685fb3df1d322d

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I66f8ee347f6c01a8a154245b10a1d93539ea13b8
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49673
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>