LU-6142 lustre: convert IFTODT to S_DT

LU-6142 lustre: convert IFTODT to S_DT

In Linux v5.1-rc1~141^2~1 introduced include/linux/fs_types.h which
adds macros for manipulating file types, including S_DT() which
does what the userpsace IFTODT() macro does.

So change kernel code to use S_DT() instead of IFTODT(), and provide
definitions for kernels which don't yet have this file.

fs_types.h is included by fs.h, so we don't need to explicitly include
it anywhere.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: If001f7e7a97992af690222b7524770c5e4b7003d
Reviewed-on: https://review.whamcloud.com/40641
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>