LU-5894 mds: allow 2.4/2.5 clients create remote dir

LU-5894 mds: allow 2.4/2.5 clients create remote dir

MDS will only return ENOTSUPP if old client (2.4/2.5) tries
to create striped dir with stripe count > 1, so it can still
create remote directory on the new MDS (>= 2.6).

Change-Id: I25c90ae793f91eed032949d26fd5e7fc41801e4f
Signed-off-by: Wang Di <di.wang@intel.com>
Reviewed-on: http://review.whamcloud.com/12715
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>