AK+Kernel: Avoid double memory clearing of HashTable buckets

Since the allocated memory is going to be zeroed immediately anyway,
let's avoid redundantly scrubbing it with MALLOC_SCRUB_BYTE just before
that.

The latest versions of gcc and Clang can automatically do this malloc +
memset -> calloc optimization, but I've seen a couple of places where it
failed to be done.

This commit also adds a naive kcalloc function to the kernel that
doesn't (yet) eliminate the redundancy like the userland does.

AK/HashTable.h

@@ -461,12 +461,11 @@ private:
         auto old_capacity = m_capacity;
         Iterator old_iter = begin();
 
-        auto* new_buckets = kmalloc(size_in_bytes(new_capacity));
+        auto* new_buckets = kcalloc(1, size_in_bytes(new_capacity));
         if (!new_buckets)
             return Error::from_errno(ENOMEM);
 
         m_buckets = (BucketType*)new_buckets;
-        __builtin_memset(m_buckets, 0, size_in_bytes(new_capacity));
 
         m_capacity = new_capacity;
         m_deleted_count = 0;

AK/kmalloc.h

@@ -15,6 +15,7 @@
 #    include <new>
 #    include <stdlib.h>
 
+#    define kcalloc calloc
 #    define kmalloc malloc
 #    define kmalloc_good_size malloc_good_size
 

Kernel/Heap/kmalloc.cpp

@@ -449,6 +449,18 @@ void* kmalloc(size_t size)
     return ptr;
 }
 
+void* kcalloc(size_t count, size_t size)
+{
+    if (Checked<size_t>::multiplication_would_overflow(count, size))
+        return nullptr;
+    size_t new_size = count * size;
+    auto* ptr = kmalloc(new_size);
+    // FIXME: Avoid redundantly scrubbing the memory in kmalloc()
+    if (ptr)
+        memset(ptr, 0, new_size);
+    return ptr;
+}
+
 void kfree_sized(void* ptr, size_t size)
 {
     if (!ptr)

Kernel/Heap/kmalloc.h

@@ -72,6 +72,7 @@ void operator delete[](void* ptrs) noexcept DISALLOW("All deletes in the kernel
 void operator delete[](void* ptr, size_t) noexcept;
 
 [[gnu::malloc, gnu::alloc_size(1)]] void* kmalloc(size_t);
+[[gnu::malloc, gnu::alloc_size(1, 2)]] void* kcalloc(size_t, size_t);
 
 [[gnu::malloc, gnu::alloc_size(1), gnu::alloc_align(2)]] void* kmalloc_aligned(size_t size, size_t alignment);