LU-13004 lnet: always pass struct lnet_md by reference.

LU-13004 lnet: always pass struct lnet_md by reference.

Both LNetMDAttach and LNetMDBind expected a struct lnet_md to be
passed by value.  This requires copying the data structure onto the
stack, which is a waste of stack space and brings no value.

So change them to expect a reference, and declare it 'const' to be
sure it doesn't get changed.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I343797d1e70cc85fde92d544e56536e982e02973
Reviewed-on: https://review.whamcloud.com/37853
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>