Kernel: Don't remap already non-writable regions when they become CoW

The only purpose of the remap() in Region::try_clone() is to ensure
non-writable page table entries for CoW regions. If a region is already
non-writable, there's no need to waste time updating the page tables.

Kernel/Memory/Region.cpp

@@ -94,7 +94,8 @@ ErrorOr<NonnullOwnPtr<Region>> Region::try_clone()
     auto vmobject_clone = TRY(vmobject().try_clone());
 
     // Set up a COW region. The parent (this) region becomes COW as well!
-    remap();
+    if (is_writable())
+        remap();
 
     OwnPtr<KString> clone_region_name;
     if (m_name)