staging: lustre: 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.

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>