LU-12021 lsom: Add an OBD_CONNECT2_LSOM connect flag

LU-12021 lsom: Add an OBD_CONNECT2_LSOM connect flag

Add an OBD_CONNECT2_LSOM connect flag so that clients do not send
MDS_ATTR_LSIZE and MDS_ATTR_LBLOCKS flags to the old servers that
do not support them.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I266c74e56c2cb1462e204d6fd4f1399f10621416
Reviewed-on: https://review.whamcloud.com/34343
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>