LU-11213 uapi: reserve connect flag for plain layout

LU-11213 uapi: reserve connect flag for plain layout

Reserve OBD_CONNECT2_PLAIN_LAYOUT flag, so that client supporting
plain layout won't enable plain layout if MDT doesn't support,
and in contrary, MDT supporting plain layout won't send such layout
to client that doesn't support.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ia629e17d83b5b48c94518de428e5abd79e5a37f0
Reviewed-on: https://review.whamcloud.com/34656
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>