LU-13814 llite: refactor ll_direct_rw_pages

LU-13814 llite: refactor ll_direct_rw_pages

ll_direct_rw_pages has some oddities in the control flow,
which make it a little harder to understand.  Clean those
up so it's easier to modify.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I64b4639df948556da03824a71b4b30806deced0d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52399
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>