Let mmap allocate a block of memory that's wide enough to use with
MADV_WIPEONFORK, madvise granularity is the current system page size
(using a static buffer of mem.page_size bytes would be wrong, that's the
minimum page size).
As a result, we don't zero some random chunk of memory every time we
fork the process.
Fixes#7609
b7eab32f42