Kernel: Make file description lookup return KResultOr

Instead of checking it at every call site (to generate EBADF), we make
file_description(fd) return a KResultOr<NonnullRefPtr<FileDescription>>.

This allows us to wrap all the calls in TRY(). :^)

The only place that got a little bit messier from this is sys$mount(),
and there's a whole bunch of things there in need of cleanup.

Kernel/Process.h

@@ -676,8 +676,7 @@ public:
             m_fds_metadatas.clear();
         }
 
-        // FIXME: Consider to remove this somehow
-        RefPtr<FileDescription> file_description(int fd) const;
+        KResultOr<NonnullRefPtr<FileDescription>> file_description(int fd) const;
 
     private:
         FileDescriptions() = default;