[PPPoE]: move lock_sock() in pppoe_sendmsg() to the right location

and the last one for now: Acquire the sock lock in pppoe_sendmsg()
before accessing the sock - and in particular avoid releasing the lock
even though it hasn't been acquired.

Signed-off-by: Florian Zumbiehl <florz@florz.de>
Signed-off-by: David S. Miller <davem@davemloft.net>