LU-13004 osp: break read request into pages.

LU-13004 osp: break read request into pages.

Rather than breaking up a read request into arbitrarily
sized (4K) pieces of memory in virtual address space,
break it up into pages (which might be 64K) and
use a kiov rather than kvec to manage them.

This is a step towards removing kvec suport and
standardizing on kiov.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: If688764c53066a9c4db212682085fa899d4dde1b
Reviewed-on: https://review.whamcloud.com/36825
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>