LU-12613 ptlrpc: check buffer length in lustre_msg_string()

LU-12613 ptlrpc: check buffer length in lustre_msg_string()

Check buffer length in lustre_msg_string() in case of any invalid
access.

Change-Id: I286000db16384938a594bd8d104e5f3d0fff585a
Reported-by: Alibaba Cloud <yunye.ry@alibaba-inc.com>
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35932
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Yunye Ry <yunye.ry@alibaba-inc.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>