Add missing read barriers in __cursor_skip_prev

    /*

     * Find the first node up the search stack that does not move.

     *

     * The depth of the current item must be at least this level, since we

     * see it in that many levels of the stack.

     *

     * !!! Watch these loops carefully: they all rely on the value of i,

     * and the exit conditions to end up with the right values are

     * non-trivial.

     */

    ins = NULL; /* -Wconditional-uninitialized */

    for (i = 0; i < WT_SKIP_MAXDEPTH - 1; i++)

        if ((ins = PREV_INS(cbt, i + 1)) != current)

            break;

    /*

     * Find a starting point for the new search. That is either at the non-moving node if we found a

     * valid node, or the beginning of the next list down that is not the current node.

     *

     * Since it is the beginning of a list, and we know the current node is has a skip depth at

     * least this high, any node we find must sort before the current node.

     */

    if (ins == NULL || ins == current)

        for (; i >= 0; i--) {

            cbt->ins_stack[i] = NULL;

            cbt->next_stack[i] = NULL;

            ins = cbt->ins_head->head[i];

            if (ins != NULL && ins != current)

                break;

        }

       else { /* Drop down a level */

            cbt->ins_stack[i] = &ins->next[i];

            cbt->next_stack[i] = ins->next[i];

            --i;

        }