Kernel: Use non-locking {Nonnull,}RefPtr for OpenFileDescription

This patch switches away from {Nonnull,}LockRefPtr to the non-locking
smart pointers throughout the kernel.

I've looked at the handful of places where these were being persisted
and I don't see any race situations.

Note that the process file descriptor table (Process::m_fds) was already
guarded via MutexProtected.

Kernel/FileSystem/File.cpp

@@ -15,7 +15,7 @@ namespace Kernel {
 File::File() = default;
 File::~File() = default;
 
-ErrorOr<NonnullLockRefPtr<OpenFileDescription>> File::open(int options)
+ErrorOr<NonnullRefPtr<OpenFileDescription>> File::open(int options)
 {
     auto description = OpenFileDescription::try_create(*this);
     if (!description.is_error()) {