LU-12678 lnet: make "struct lnet_lnd" always "const".

LU-12678 lnet: make "struct lnet_lnd" always "const".

Every place where "struct lnet_lnd" appears, "const" is
added in front. Now all those structs can be in read-only
memory which is generally more secure.

Linux-commit 07499855083e ("lnet: make "struct lnet_lnd"
                            always "const".")

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I54a73d5b12de8c6b9a98182577c3c30d05c00222
Reviewed-on: https://review.whamcloud.com/36832
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Amir Shehata <ashehata@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>