LU-13004 lnet: always put a page list into struct lnet_libmd

LU-13004 lnet: always put a page list into struct lnet_libmd

'struct lnet_libmd' is only created in lnet_md_build().  It can be
given a list of pages or a virtual address.  In the latter case, the
memory will eventually be split into a list of pages.  It is cleaner
to split it into a list of pages early so that all lower levels only
need to handle one type: a page list.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Iaeb4f18f7ac63986e6fed259945e8594ea1b9f25
Reviewed-on: https://review.whamcloud.com/37842
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.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>