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/ioctl.cpp

@@ -13,9 +13,7 @@ namespace Kernel {
 KResultOr<FlatPtr> Process::sys$ioctl(int fd, unsigned request, FlatPtr arg)
 {
     VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this)
-    auto description = fds().file_description(fd);
-    if (!description)
-        return EBADF;
+    auto description = TRY(fds().file_description(fd));
     if (request == FIONBIO) {
         description->set_blocking(arg == 0);
         return KSuccess;