Kernel: Stop using *LockRefPtr for FileSystem pointers

There was only one permanent storage location for these: as a member
in the Mount class.

That member is never modified after Mount initialization, so we don't
need to worry about races there.

Kernel/FileSystem/DevPtsFS/FileSystem.h

@@ -20,7 +20,7 @@ class DevPtsFS final : public FileSystem {
 
 public:
     virtual ~DevPtsFS() override;
-    static ErrorOr<NonnullLockRefPtr<FileSystem>> try_create();
+    static ErrorOr<NonnullRefPtr<FileSystem>> try_create();
 
     virtual ErrorOr<void> initialize() override;
     virtual StringView class_name() const override { return "DevPtsFS"sv; }