kernel/vm_manager: Reserve memory ahead of time for slow path in MergeAdjacentVMA

Avoids potentially expensive (depending on the size of the memory block)
allocations by reserving the necessary memory before performing both
insertions. This avoids scenarios where the second insert may cause a
reallocation to occur.

src/core/hle/kernel/vm_manager.cpp

@@ -765,11 +765,14 @@ void VMManager::MergeAdjacentVMA(VirtualMemoryArea& left, const VirtualMemoryAre
             // Fast case: left is an entire backing block.
             left.backing_block->insert(left.backing_block->end(), right_begin, right_end);
         } else {
+            // Slow case: make a new memory block for left and right.
             const auto left_begin = left.backing_block->begin() + left.offset;
             const auto left_end = left_begin + left.size;
+            const auto left_size = static_cast<std::size_t>(std::distance(left_begin, left_end));
+            const auto right_size = static_cast<std::size_t>(std::distance(right_begin, right_end));
 
-            // Slow case: make a new memory block for left and right.
             auto new_memory = std::make_shared<PhysicalMemory>();
+            new_memory->reserve(left_size + right_size);
             new_memory->insert(new_memory->end(), left_begin, left_end);
             new_memory->insert(new_memory->end(), right_begin, right_end);