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

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

Kernel/FileSystem/FIFO.cpp

@@ -104,7 +104,7 @@ bool FIFO::can_read(const OpenFileDescription&, u64) const
     return !m_buffer->is_empty() || !m_writers;
 }
 
-bool FIFO::can_write(const OpenFileDescription&, size_t) const
+bool FIFO::can_write(const OpenFileDescription&, u64) const
 {
     return m_buffer->space_for_writing() || !m_readers;
 }