[PPPOL2TP]: Make locking calls softirq-safe

Fix locking issues in the pppol2tp driver which can cause a kernel
crash on SMP boxes. There were two problems:-

1. The driver was violating read_lock() and write_lock() scheduling
   rules because it wasn't using softirq-safe locks in softirq
   contexts. So we now consistently use the _bh variants of the lock
   functions.

2. The driver was calling sk_dst_get() in pppol2tp_xmit() which was
   taking sk_dst_lock in softirq context. We now call __sk_dst_get().

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>