From: Christoph Hellwig <hch@lst.de>

Currently the tty code abuses tty_driver.name as the prefix for the devfs
names of the ttys.  This is a very bad idea because it means the tty name
changes depending on whether devfs is enabled or not, leading to different
names in /proc/tty/ depending on whether we have devfs or not (and not
whether it actually is mounted!) and a huge amount of ifdefs.

The patch below adds a .devfs_name member instead, similar to the block
device changes a few weeks ago.