[fix] mappages: potential memory leak

[yztz]

In mappages, if size > 1 * PAGESIZE or va not page-aligned(i.e. need to map more than one page), it will lead to a memory leak with a midway kalloc failure. When mappages returns -1, it will try to call uvmfree->freewalk to free the page-table pages and may lead to a panic because of an mapped page. In the unmodified version, maapages always successes or is only called with 1 * PAGESIZE, so this problem doesn't rise.