Kernel+LibC: Make get_dir_entries syscall retriable

The get_dir_entries syscall failed if the serialized form of all the
directory entries together was too large to fit in its temporary buffer.

Now the kernel uses a fixed size buffer, that is flushed to an output
buffer when it is full. If this flushing operation fails because there
is not enough space available, the syscall will return -EINVAL. That
error code is then used in userspace as a signal to allocate a larger
buffer and retry the syscall.

AK/MemoryStream.h

@@ -129,6 +129,7 @@ public:
 
     size_t size() const { return m_offset; }
     size_t remaining() const { return m_bytes.size() - m_offset; }
+    void reset() { m_offset = 0; }
 
 private:
     size_t m_offset { 0 };