pppoe: remove redundant BUG_ON() check in pppoe_pernet

Passing NULL to pppoe_pernet causes a crash via BUG_ON.
Dereferencing net in net_generici() also has the same effect. This patch
removes the redundant BUG_ON check on the same parameter.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>