[NEIGH]: Ensure that pneigh_lookup is protected with RTNL

The pnigh_lookup is used to lookup proxy entries and to 
create them in case lookup failed. 

However, the "creation" code does not perform the re-lookup
after GFP_KERNEL allocation. This is done because the code
is expected to be protected with the RTNL lock, so add the 
assertion (mainly to address future questions from new network 
developers like me :) ).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>