LU-12400 osd-ldiskfs: get rid of legacy 'get_ds()' function

LU-12400 osd-ldiskfs: get rid of legacy 'get_ds()' function

get rid of legacy 'get_ds()' function

Every in-kernel use of this function defined it to KERNEL_DS
(either as an actual define, or as an inline function).

Linux-commit: 736706bee3298208343a76096370e4f6a5c55915

Test-Parameters: trivial
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I3aabe74802f1a953b140728f22c83125dae270c3
Reviewed-on: https://review.whamcloud.com/35238
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>