LU-12514 target: move server mount code to target layer

LU-12514 target: move server mount code to target layer

Currently the server mount code for lustre_tgt is all in obdclass. If
we change the stack to initialize the LNet / ptlrpc layer after mounting
then we will end up with modular circular dependencies. To avoid this
move all the lustre_tgt mounting code to the target layer. This way the
mounting code can use both ptlrpc and obdclass module routiens. Also include
MODULE_ALAIS("lustre_tgt") so mount -t lustre_tgt will load ptlrpc which
contains the target layer.

Change-Id: I392602e8fd18d001cb97b05b909c366ba5b8fa82
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/47160
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>