LU-11427 llite: optimize read on open pages

LU-11427 llite: optimize read on open pages

Current read-on-open implementation does allocate cl_page after data
are piggied back by open request, which is expensive and not
necessary.

This patch improves the case by just adding the pages into page cache.
As long as those pages will be discarded at lock revocation, there
should be no concerns.

Signed-off-by: Jinshan Xiong <jinshan.xiong@uber.com>
Change-Id: Idef1b70483e3780790ba5b95c26ef2d4141add5f
Reviewed-on: https://review.whamcloud.com/33234
Tested-by: Jenkins
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>