LU-8400 mdd: remove OBD_IOC_GET_MNTOPT

LU-8400 mdd: remove OBD_IOC_GET_MNTOPT

This ioctl was only used between MDT and MDD.

Replace it, as well as mdo_maxeasize_get by a generic mdo_dtconf_get
function.

Change-Id: Idb080007a0b7311d6fc95f06cf24db8e9c3265d6
Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-on: https://review.whamcloud.com/28777
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>