This patch prevents execution of hvc_write() and hvc_hangup() after the tty
layer has executed a final hvc_close() against a device.  This patch
provides a better method than was previously used.  tty->driver_data is no
longer invalidated so we'll no longer get oopses when the tty layer allows
late hangup() and write() operations.

- Removed silly tty->driver_data = NULL; from hvc_close which prevents
  possible oops in hvc_write() and hvc_hangup() due to improperly acting
  ldisc close ordering.

- Added hp->count <= 0 check to hvc_write() and hvc_hangup() to prevent
  execution of these function after hvc_close() has been invoked by the tty
  layer.  Same tty ldisc issues as above are the reason.

- Added some comments to clarify the situation.

- Awaiting a forth coming patch from Alan Cox which should clean up the
  close ordering and prevent the late hangup and write ops from happening.
Signed-off-by: Ryan S. Arnold <rsa@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>