Kernel: Assert that copy_to/from_user() are called with user addresses

This will panic the kernel immediately if these functions are misused
so we can catch it and fix the misuse.

This patch fixes a couple of misuses:

    - create_signal_trampolines() writes to a user-accessible page
      above the 3GB address mark. We should really get rid of this
      page but that's a whole other thing.

    - CoW faults need to use copy_from_user rather than copy_to_user
      since it's the *source* pointer that points to user memory.

    - Inode faults need to use memcpy rather than copy_to_user since
      we're copying a kernel stack buffer into a quickmapped page.

This should make the copy_to/from_user() functions slightly less useful
for exploitation. Before this, they were essentially just glorified
memcpy() with SMAP disabled. :^)

Kernel/VM/MemoryManager.h

@@ -214,3 +214,10 @@ inline bool is_user_address(VirtualAddress vaddr)
 {
     return vaddr.get() < 0xc0000000;
 }
+
+inline bool is_user_range(VirtualAddress vaddr, size_t size)
+{
+    if (vaddr.offset(size) < vaddr)
+        return false;
+    return is_user_address(vaddr) && is_user_address(vaddr.offset(size));
+}