Kernel: Fix subtle race condition in sys$write implementation

There is a slight race condition in our implementation of write().
We call File::can_write() before attempting to write to it (blocking if
it returns false). If it returns true, we assume that we can write to
the file, and our code assumes that File::write() cannot possibly fail
by being blocked. There is, however, the rare case where another process
writes to the file and prevents further writes in between the call to
Files::can_write() and File::write() in the first process. This would
result in the first process calling File::write() when it cannot be
written to.

We fix this by adding a mechanism for File::can_write() to signal that
it was blocked, making it the responsibilty of File::write() to check
whether it can write and then finally making sys$write() check if the
write failed due to it being blocked.

Kernel/Devices/SerialDevice.h

@@ -135,6 +135,7 @@ private:
     bool m_break_enable { false };
     u8 m_modem_control { 0 };
     bool m_last_put_char_was_carriage_return { false };
+    SpinLock<u8> m_serial_lock;
 };
 
 }