LU-13004 osp: use correct page count in osp_prep_update_req

LU-13004 osp: use correct page count in osp_prep_update_req

A fix that went into patchset 3 of
 https://review.whamcloud.com/#/c/36828/3
disappeared in patchset 5.
We should restore it.

Specifically, 'page_count' should be a count of pages,
but it is currently a count of the bytes in all the pages.

Fixes: f32fbf189fab ("LU-13004 osp: use KIOV in osp_prep_update_req")
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ic8dcdac414d16b4f2c1c6e0367d496de7e0a8cff
Reviewed-on: https://review.whamcloud.com/37587
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>