LU-13004 lnet: merge lnet_md_alloc into lnet_md_build.

lnet_md_alloc is only called twice, each time immediately before a
call to lnet_md_build(), and these are the only calls to
lnet_md_build().

So simplify the code by merging lnet_md_alloc into lnet_md_build.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I13de6d2b225052832d86d4ebe5bf9c2eae945720
Reviewed-on: https://review.whamcloud.com/37841
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>