tty: fall back to N_NULL if switching to N_TTY fails during hangup

We have seen NULL-pointer dereference crashes in tty->disc_data when the
N_TTY fallback driver failed to open during hangup.  The immediate cause
of this open to fail has been addressed in the preceding patch to
vmalloc(), but this code could be more robust.

As Alan pointed out in commit 8a8dabf2dd68 ("tty: handle the case where
we cannot restore a line discipline"), the N_TTY driver, historically
the safe fallback that could never fail, can indeed fail, but the
surrounding code is not prepared to handle this.  To avoid crashes he
added a new N_NULL driver to take N_TTY's place as the last resort.

Hook that fallback up to the hangup path.  Update tty_ldisc_reinit() to
reflect the reality that n_tty_open can indeed fail.

Link: http://lkml.kernel.org/r/20171004185959.GC2136@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alan Cox <alan@llwyncelyn.cymru>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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