Kernel: Simplify Mount internals

- The host custody never changes after initialization, so there's no
  need to protect it with a spinlock.

- To enforce the fact that some members don't change after
  initialization, make them const.

Kernel/FileSystem/Mount.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2018-2023, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -11,47 +11,41 @@
 
 namespace Kernel {
 
-Mount::Mount(NonnullRefPtr<FileSystem> guest_fs, Custody* host_custody, int flags)
-    : m_guest(guest_fs->root_inode())
-    , m_guest_fs(guest_fs)
-    , m_host_custody(host_custody)
+Mount::Mount(NonnullRefPtr<FileSystem> guest_fs, RefPtr<Custody> host_custody, int flags)
+    : m_guest_fs(move(guest_fs))
+    , m_guest(m_guest_fs->root_inode())
+    , m_host_custody(move(host_custody))
     , m_flags(flags)
 {
 }
 
-Mount::Mount(Inode& source, Custody& host_custody, int flags)
-    : m_guest(source)
-    , m_guest_fs(source.fs())
-    , m_host_custody(host_custody)
+Mount::Mount(NonnullRefPtr<Inode> source, NonnullRefPtr<Custody> host_custody, int flags)
+    : m_guest_fs(source->fs())
+    , m_guest(move(source))
+    , m_host_custody(move(host_custody))
     , m_flags(flags)
 {
 }
 
 ErrorOr<NonnullOwnPtr<KString>> Mount::absolute_path() const
 {
-    return m_host_custody.with([&](auto& host_custody) -> ErrorOr<NonnullOwnPtr<KString>> {
-        if (!host_custody)
-            return KString::try_create("/"sv);
-        return host_custody->try_serialize_absolute_path();
-    });
+    if (!m_host_custody)
+        return KString::try_create("/"sv);
+    return m_host_custody->try_serialize_absolute_path();
 }
 
 RefPtr<Inode> Mount::host()
 {
-    return m_host_custody.with([](auto& host_custody) -> RefPtr<Inode> {
-        if (!host_custody)
-            return nullptr;
-        return &host_custody->inode();
-    });
+    if (!m_host_custody)
+        return nullptr;
+    return m_host_custody->inode();
 }
 
 RefPtr<Inode const> Mount::host() const
 {
-    return m_host_custody.with([](auto& host_custody) -> RefPtr<Inode const> {
-        if (!host_custody)
-            return nullptr;
-        return &host_custody->inode();
-    });
+    if (!m_host_custody)
+        return nullptr;
+    return m_host_custody->inode();
 }
 
 }

Kernel/FileSystem/Mount.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2018-2023, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -17,11 +17,13 @@ namespace Kernel {
 
 class VirtualFileSystem;
 class Mount {
+    AK_MAKE_NONCOPYABLE(Mount);
+    AK_MAKE_NONMOVABLE(Mount);
     friend class VirtualFileSystem;
 
 public:
-    Mount(NonnullRefPtr<FileSystem>, Custody* host_custody, int flags);
-    Mount(Inode& source, Custody& host_custody, int flags);
+    Mount(NonnullRefPtr<FileSystem>, RefPtr<Custody> host_custody, int flags);
+    Mount(NonnullRefPtr<Inode> source, NonnullRefPtr<Custody> host_custody, int flags);
 
     RefPtr<Inode const> host() const;
     RefPtr<Inode> host();
@@ -38,10 +40,10 @@ public:
     void set_flags(int flags) { m_flags = flags; }
 
 private:
-    NonnullRefPtr<Inode> m_guest;
-    NonnullRefPtr<FileSystem> m_guest_fs;
-    SpinlockProtected<RefPtr<Custody>, LockRank::None> m_host_custody;
-    int m_flags;
+    NonnullRefPtr<FileSystem> const m_guest_fs;
+    NonnullRefPtr<Inode> const m_guest;
+    RefPtr<Custody> const m_host_custody;
+    int m_flags { 0 };
 
     IntrusiveListNode<Mount> m_vfs_list_node;
 };