In _malloc_init(), round up the region being mprotected RW to the malloc

In _malloc_init(), round up the region being mprotected RW to the malloc
page size, rather than relying upon mprotect to round up to the actual mmu
page size.

This repairs malloc operation on systems where the malloc page size
(1 << _MAX_PAGE_SHIFT) is larger than the mmu page size.

ok otto@