LU-15803 sec: correctly handle page lock in ll_io_zero_page

LU-15803 sec: correctly handle page lock in ll_io_zero_page

In ll_io_zero_page(), we need to make sure we have locked the page,
and it is up-to-date, before zeroing. So modify ll_io_read_page()
behavior to not disown the clpage for our use case. It avoids being
exposed to concurrent modifications.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I58e4cf80374a798c9c4302364cf2fb39da9033bb
Reviewed-on: https://review.whamcloud.com/47170
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>