LU-12514 llite: move client mounting from obdclass to llite

LU-12514 llite: move client mounting from obdclass to llite

Mounting a lustre client is currently handled
in obdclass, using services from llite.
This requires obdclass to load the llite module
and set up inter-module linkage.

The purpose of this was for common code to support both
client and server mounts.  This isn't really a good idea
and need to go. For lustre servers we already use a
separate filesystem type.

So move the mounting code from obdclass/obd_mount to llite/super25
and remove the inter-module linkages.
Add some EXPORT_SYMBOL() so that llite can access some helpers
that remain in obdclass.

Linux-commit: a989830c88149511ee840356d9c1b34304bac576

Lustre-change: https://review.whamcloud.com/37693
Lustre-commit: 53fa81765750e38f7879ed5092fd729c1bdc8a0f

Change-Id: Ia33bd55a042f90b178156c745a8072b516f00568
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51315
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>