n_tty: Fix apparent order of echoed output

n_tty: Fix apparent order of echoed output

With block processing of echoed output, observed output order is still
required. Push completed echoes and echo commands prior to output.

Introduce echo_mark echo buffer index, which tracks completed echo
commands; ie., those submitted via commit_echoes but which may not
have been committed. Ensure that completed echoes are output prior
to subsequent terminal writes in process_echoes().

Fixes newline/prompt output order in cooked mode shell.

Cc: <[email protected]> # 3.12.x : 39434ab n_tty: Fix missing newline echo
Reported-by: Karl Dahlke <[email protected]>
Reported-by: Mikulas Patocka <[email protected]>
Signed-off-by: Peter Hurley <[email protected]>
Tested-by: Karl Dahlke <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>