n_tty: Don't lose characters when PARMRK is enabled

When PARMRK is set and large transfers of characters that will get
marked are being received, n_tty could drop data silently (i.e.
without reporting any error to the client). This is because
characters have the potential to take up to three bytes in the line
discipline (when they get marked with parity or framing errors), but
the amount of free space reported to tty_buffer flush_to_ldisc (via
tty->receive_room) is based on the pre-marked data size.

With this patch, the n_tty layer will no longer assume that each byte
will only take up one byte in the line discipline. Instead, it will
make an overly conservative estimate that each byte will take up
three bytes in the line discipline when PARMRK is set.

Signed-off-by: Jaeden Amero <jaeden.amero@ni.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>