Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Kernel/FileSystem/BlockBasedFileSystem.cpp

@@ -290,7 +290,7 @@ void BlockBasedFS::flush_specific_block_if_needed(unsigned index)
             file_description().seek(base_offset, SEEK_SET);
             // FIXME: Should this error path be surfaced somehow?
             auto entry_data_buffer = UserOrKernelBuffer::for_kernel_buffer(entry.data);
-            (void)file_description().write(entry_data_buffer, block_size());
+            [[maybe_unused]] auto rc = file_description().write(entry_data_buffer, block_size());
             cleaned_entries.append(&entry);
         }
     });
@@ -311,7 +311,7 @@ void BlockBasedFS::flush_writes_impl()
         file_description().seek(base_offset, SEEK_SET);
         // FIXME: Should this error path be surfaced somehow?
         auto entry_data_buffer = UserOrKernelBuffer::for_kernel_buffer(entry.data);
-        (void)file_description().write(entry_data_buffer, block_size());
+        [[maybe_unused]] auto rc = file_description().write(entry_data_buffer, block_size());
         ++count;
     });
     cache().mark_all_clean();

Kernel/FileSystem/FileDescription.cpp

@@ -71,7 +71,7 @@ FileDescription::~FileDescription()
     if (is_fifo())
         static_cast<FIFO*>(m_file.ptr())->detach(m_fifo_direction);
     // FIXME: Should this error path be observed somehow?
-    (void)m_file->close();
+    [[maybe_unused]] auto rc = m_file->close();
     m_inode = nullptr;
 }
 

Kernel/FileSystem/Plan9FileSystem.cpp

@@ -710,7 +710,7 @@ Plan9FSInode::~Plan9FSInode()
     Plan9FS::Message clunk_request { fs(), Plan9FS::Message::Type::Tclunk };
     clunk_request << fid();
     // FIXME: Should we observe this  error somehow?
-    (void)fs().post_message_and_explicitly_ignore_reply(clunk_request);
+    [[maybe_unused]] auto rc = fs().post_message_and_explicitly_ignore_reply(clunk_request);
 }
 
 KResult Plan9FSInode::ensure_open_for_mode(int mode)
@@ -909,7 +909,7 @@ KResult Plan9FSInode::traverse_as_directory(Function<bool(const FS::DirectoryEnt
                 Plan9FS::Message close_message { fs(), Plan9FS::Message::Type::Tclunk };
                 close_message << clone_fid;
                 // FIXME: Should we observe this error?
-                (void)fs().post_message_and_explicitly_ignore_reply(close_message);
+                [[maybe_unused]] auto rc = fs().post_message_and_explicitly_ignore_reply(close_message);
                 return result;
             }
         }
@@ -942,7 +942,7 @@ KResult Plan9FSInode::traverse_as_directory(Function<bool(const FS::DirectoryEnt
         Plan9FS::Message close_message { fs(), Plan9FS::Message::Type::Tclunk };
         close_message << clone_fid;
         // FIXME: Should we observe this error?
-        (void)fs().post_message_and_explicitly_ignore_reply(close_message);
+        [[maybe_unused]] auto rc = fs().post_message_and_explicitly_ignore_reply(close_message);
         return result;
     } else {
         // TODO

Kernel/FileSystem/ProcFS.cpp

@@ -1647,9 +1647,8 @@ KResult ProcFSInode::add_child(Inode&, const StringView&, mode_t)
     return KResult(-EPERM);
 }
 
-KResult ProcFSInode::remove_child(const StringView& name)
+KResult ProcFSInode::remove_child([[maybe_unused]] const StringView& name)
 {
-    (void)name;
     return KResult(-EPERM);
 }