Kernel/FileSystem: Simplify even more the mount syscall

As with the previous commit, we put a distinction between filesystems
that require a file description and those which don't, but now in a much
more readable mechanism - all initialization properties as well as the
create static method are grouped to create the FileSystemInitializer
structure. Then when we need to initialize an instance, we iterate over
a table of these structures, checking for matching structure and then
validating the given arguments from userspace against the requirements
to ensure we can create a valid instance of the requested filesystem.

Kernel/FileSystem/ISO9660FileSystem.h

@@ -305,7 +305,7 @@ public:
         }
     };
 
-    static ErrorOr<NonnullRefPtr<ISO9660FS>> try_create(OpenFileDescription&);
+    static ErrorOr<NonnullRefPtr<FileSystem>> try_create(OpenFileDescription&);
 
     virtual ~ISO9660FS() override;
     virtual ErrorOr<void> initialize() override;