tty: don't panic on OOM in tty_set_ldisc()

tty: don't panic on OOM in tty_set_ldisc()

If tty_ldisc_open() fails in tty_set_ldisc(), it tries to go back
to the old discipline or N_TTY. But that can fail as well, in such
case it panics. This is not a graceful way to handle OOM.

Leave ldisc==NULL if all attempts fail instead.
Also use existing tty_ldisc_reinit() helper function instead of
tty_ldisc_restore(). Also don't WARN/BUG in tty_ldisc_reinit()
if N_TTY fails, which would have the same net effect of bringing
kernel down on OOM. Instead print a single line message about
what has happened.

Signed-off-by: Dmitry Vyukov <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Peter Hurley <[email protected]>
Cc: One Thousand Gnomes <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>