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/Syscalls/get_dir_entries.cpp

@@ -15,9 +15,7 @@ KResultOr<FlatPtr> Process::sys$get_dir_entries(int fd, Userspace<void*> user_bu
     REQUIRE_PROMISE(stdio);
     if (user_size > NumericLimits<ssize_t>::max())
         return EINVAL;
-    auto description = fds().file_description(fd);
-    if (!description)
-        return EBADF;
+    auto description = TRY(fds().file_description(fd));
     auto buffer = UserOrKernelBuffer::for_user_buffer(user_buffer, static_cast<size_t>(user_size));
     if (!buffer.has_value())
         return EFAULT;