LU-8837 lustre: move lu_tgt_pool out of obd_target.h

LU-8837 lustre: move lu_tgt_pool out of obd_target.h

'struct lu_tgt_pool' is the only part of obd_target.h that is needed
on the client.
So move it and related declarations to lu_object.h.
Then obd_target.h does not need to be included by lu_object.h,
and it will only be included server-side;

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ia578d766fb7fae6ae2fbcfa651fe5b5264b1ff14
Reviewed-on: https://review.whamcloud.com/41951
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>