LU-13799 llite: Remove unnecessary page get/put

LU-13799 llite: Remove unnecessary page get/put

Part of the aio cleanup code has the slightly strange
behavior of doing get on every page before calling page
cleanup, then doing a put after.

This was required because we call cl_page_list_del before
calling cl_page_delete, and cl_page_list_del was holding
the last reference on the page struct.

If we reverse the order, then we don't need the extra
get/put to keep the pages live.  This should save
significant CPU time in the ptlrpcd threads when finishing
i/o, since this removes a get/put on every page.

Lustre-change: https://review.whamcloud.com/44293
Lustre-commit: c2e94f08cf3ff000b350faf61b6d25ebbad7970e

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ia9e7bf4a331a5220bfb1eab43493ac5fda59c611
Reviewed-on: https://review.whamcloud.com/44688
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>