Kernel: Implement proper memory region splitting

Memory regions are now splitted when they get munmapped, mprotected, or
mmapped with MAP_FIXED. This is used by couple of ports, and without
this we were just leaking up memory or straight up crashing programs.

kernel/include/kernel/Memory/MemoryBackedRegion.h

@@ -15,6 +15,7 @@ namespace Kernel
 		~MemoryBackedRegion();
 
 		BAN::ErrorOr<BAN::UniqPtr<MemoryRegion>> clone(PageTable& new_page_table) override;
+		BAN::ErrorOr<BAN::UniqPtr<MemoryRegion>> split(size_t offset) override;
 
 		BAN::ErrorOr<void> msync(vaddr_t, size_t, int) override { return {}; }