libroot/malloc: Increase address space reservation size to 512 MB.

The kernel will ignore the reservations if there's not enough space
left in the address space anyway.

It seems using larger reservations prevents address space fragmentation,
and so this by itself may be enough to fix #19592.

src/system/libroot/posix/malloc/openbsd/PagesAllocator.cpp

@@ -27,7 +27,7 @@ static const size_t kPageSize = B_PAGE_SIZE;
 static const size_t kLargestUsefulChunk = 512 * kPageSize;
 
 /*! Amount of virtual address space to reserve when creating new areas. */
-static const size_t kReserveAddressSpace = 128 * 1024 * 1024;
+static const size_t kReserveAddressSpace = 512 * 1024 * 1024;
 
 /*! Cache up to this many percentage points of free memory (compared to used.) */
 static const size_t kFreePercentage = 25;