Kernel: Avoid repeated memory zeroing while generating coredumps

Reuse the same buffer-full-of-zeroes for every un-paged-in page that we
dump out.

Kernel/Coredump.cpp

@@ -167,6 +167,8 @@ ErrorOr<void> Coredump::write_program_headers(size_t notes_size)
 
 ErrorOr<void> Coredump::write_regions()
 {
+ u8 zero_buffer[PAGE_SIZE] = {};
+
  for (auto& region : m_process->address_space().regions()) {
  VERIFY(!region->is_kernel());
 
@@ -180,8 +182,6 @@ ErrorOr<void> Coredump::write_regions()
 
  for (size_t i = 0; i < region->page_count(); i++) {
  auto* page = region->physical_page(i);
-
- uint8_t zero_buffer[PAGE_SIZE] = {};
  auto src_buffer = [&]() -> ErrorOr<UserOrKernelBuffer> {
  if (page)
  return UserOrKernelBuffer::for_user_buffer(reinterpret_cast<uint8_t*>((region->vaddr().as_ptr() + (i * PAGE_SIZE))), PAGE_SIZE);