n_tty: do O_ONLCR translation as a single write

When translating CR to CRNL in the n_tty line discipline, we did it as
two tty_put_char() calls.  Which works, but is stupid, and has caused
problems before too with bad interactions with the write_room() logic.
The generic USB serial driver had that problem, for example.

Now the pty layer had similar issues after being moved to the generic
tty buffering code (in commit d945cb9cce20ac7143c2de8d88b187f62db99bdc:
"pty: Rework the pty layer to use the normal buffering logic").

So stop doing the silly separate two writes, and do it as a single write
instead.  That's what the n_tty layer already does for the space
expansion of tabs (XTABS), and it means that we'll now always have just
a single write for the CRNL to match the single 'tty_write_room()' test,
which hopefully means that the next time somebody screws up buffering,
it won't cause weeks of debugging.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/n_tty.c

@@ -300,8 +300,7 @@ static int do_output_char(unsigned char c, struct tty_struct *tty, int space)
 			if (space < 2)
 				return -1;
 			tty->canon_column = tty->column = 0;
-			tty_put_char(tty, '\r');
-			tty_put_char(tty, c);
+			tty->ops->write(tty, "\r\n", 2);
 			return 2;
 		}
 		tty->canon_column = tty->column;