LU-13805 clio: Add csi_complete

LU-13805 clio: Add csi_complete

The next patch will make end_io potentially sleep, so we
need to modify how completion works to avoid holding a
spinlock over the end_io() call.

This patch is strictly supporting work for the next patch
and has been pulled out so it can be tested by itself.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Iba3388a0e09fdd0ab2f4a95f1cde96908a485cfa
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49913
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>