Patch from Robert Love.

This patch implements for_each_pgdat(pg_data_t *) which is a helper
macro to cleanup code that does a loop of the form:

        pgdat = pgdat_list;
        while(pgdat) {
	        /* ... */
	        pgdat = pgdat->node_next;
	}

and replace it with:

	for_each_pgdat(pgdat) {
		/* ... */
	}

This code is from Rik's 2.4-rmap patch and is by William Irwin.