LU-10994 clio: Remove cl_2queue_add wrapper

Remove the wrapper function cl_2queue_add() and replace all its calls in
different files with the function it wrapped. Also, comments are added
wherever necessary to make the working of function clear. Prototype of
the function is also removed from the header file as it is no longer
needed.

Linux-commit: 53f1a12768a55e53b2c40e00a8804b1edfa739b3

Change-Id: Ic746c45e3dda9fdf3f1d2f8c6204d80fec5c058f
Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/47651
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>