Kernel: Use u64 instead of size_t for File::can_read offset

This ensures offsets will not be truncated on large files on i686.

Kernel/FileSystem/FIFO.cpp

@@ -99,7 +99,7 @@ void FIFO::detach(Direction direction)
     evaluate_block_conditions();
 }
 
-bool FIFO::can_read(const OpenFileDescription&, size_t) const
+bool FIFO::can_read(const OpenFileDescription&, u64) const
 {
     return !m_buffer->is_empty() || !m_writers;
 }