Instead of detecting when filp->private_data becomes NULL in
do_tty_hangup and check_tty_count, we remove the file descriptor
from the list of descriptors associated with the tty.  We use the
same method that dentry_open() uses.

(It also cleans that up to use a "file_kill()" instead of using a dummy
"kill_list()" that leaves pointers to stale stack entries that are never
used)

In addition, we change the for() loops into real list_for_each()
or list_for_each_entry loops as appropriate.