Kernel: Support best fit allocation policy in kmalloc()

Add find_best_fit() which implements best fit allocation algorithm.
Kmalloc now uses a best fit allocation policy for large allocations.

AK/Bitmap.h

@@ -285,6 +285,35 @@ public:
         return {};
     }
 
+    Optional<size_t> find_best_fit(size_t minimum_length) const
+    {
+        size_t start = 0;
+        size_t best_region_start = 0;
+        size_t best_region_size = max_size;
+        bool found = false;
+
+        while (true) {
+            // Look for the next block which is bigger than requested length.
+            auto length_of_found_range = find_next_range_of_unset_bits(start, minimum_length, best_region_size);
+            if (length_of_found_range.has_value()) {
+                if (best_region_size > length_of_found_range.value() || !found) {
+                    best_region_start = start;
+                    best_region_size = length_of_found_range.value();
+                    found = true;
+                }
+                start += length_of_found_range.value();
+            } else {
+                // There are no ranges which can fit requested length.
+                break;
+            }
+        }
+
+        if (found) {
+            return best_region_start;
+        }
+        return {};
+    }
+
     Bitmap()
         : m_size(0)
         , m_owned(true)

Kernel/Heap/kmalloc.cpp

@@ -150,7 +150,16 @@ void* kmalloc_impl(size_t size)
     size_t chunks_needed = (real_size + CHUNK_SIZE - 1) / CHUNK_SIZE;
 
     Bitmap bitmap_wrapper = Bitmap::wrap(alloc_map, POOL_SIZE / CHUNK_SIZE);
-    auto first_chunk = bitmap_wrapper.find_first_fit(chunks_needed);
+    Optional<size_t> first_chunk;
+
+    // Choose the right politic for allocation.
+    constexpr u32 best_fit_threshold = 128;
+    if (chunks_needed < best_fit_threshold) {
+        first_chunk = bitmap_wrapper.find_first_fit(chunks_needed);
+    } else {
+        first_chunk = bitmap_wrapper.find_best_fit(chunks_needed);
+    }
+
     if (!first_chunk.has_value()) {
         klog() << "kmalloc(): PANIC! Out of memory (no suitable block for size " << size << ")";
         Kernel::dump_backtrace();