ppp: push BKL down into the driver

I've pushed it down as far as I dare at this point. Someone familiar with
the internal PPP semantics can probably push it further. Another step to
eliminating the old BKL ioctl usage.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>