cygwin: mmap: fix a fork failure with private, anonymous mappings

Rounddown incoming addr on a page boundary. Without this, we may end
up with a fork error for private, anonymous maps.  The reason is, we
use VirtualAlloc in this case which will potentially overcommit if
addr is not on a page boundary.  This isn't taken into account in
bookkeeping, but fixup_mmaps_after_fork will eventually stumble over
this when trying to reproduce the copy-on-write pages: VirtualQuery
returns a region reaching beyond the supposedly allocated address
range and from there it goes downhill.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>