Ensure all relevant btree fields are reset on checkpoint error

Running with the test program from WT-3157, and adding in a forced error in checkpoint_tree I get an assertion failure/core dump during error handling. The command is:
./test_wt2909_checkpoint_integrity -v -o 125 and the code change for the forced error is:

        WT_ERR(__wt_page_modify_init(session, btree->root.page));

        if (++ckpt_first > 1)

                WT_ERR(EIO);

        __wt_page_only_modify_set(session, btree->root.page);