Kernel: Make KBuffer::try_create_with_bytes() actually copy the bytes

KBuffers created with this API were actually just zero-filled instead
of being populated with the provided bytes.

Fixes #4493.

Kernel/KBuffer.h

@@ -63,6 +63,7 @@ public:
             return nullptr;
         if (!region->commit())
             return nullptr;
+        memcpy(region->vaddr().as_ptr(), bytes.data(), bytes.size());
         return adopt(*new KBufferImpl(region.release_nonnull(), bytes.size()));
     }