Kernel: Allocate more 8-byte slabs than anything else

We need these for PhysicalPage objects. Ultimately I'd like to get rid
of these objects entirely, but while we still have to deal with them,
let's at least handle large demand a bit better.

Kernel/Heap/SlabAllocator.cpp

@@ -79,9 +79,10 @@ void for_each_allocator(Callback callback)
 
 void slab_alloc_init()
 {
-    for_each_allocator([&](auto& allocator) {
-        allocator.init(128 * KB);
-    });
+    s_slab_allocator_8.init(384 * KB);
+    s_slab_allocator_16.init(128 * KB);
+    s_slab_allocator_32.init(128 * KB);
+    s_slab_allocator_48.init(128 * KB);
 }
 
 void* slab_alloc(size_t slab_size)