Revert "NET: Fix locking issues in PPP, 6pack, mkiss and strip line disciplines."

This reverts commit adeab1afb7de89555c69aab5ca21300c14af6369.

As Alan Cox explained, the TTY layer changes that went recently
to get rid of the tty->low_latency stuff fixes this already,
and even for -stable it's the ->low_latency changes that should
go in to fix this, rather than this patch.

Signed-off-by: David S. Miller <davem@davemloft.net>