pppoe: fix /proc/net/pppoe

If a socket is hashed in last slot of pppoe hash table (PPPOE_HASH_SIZE-1)
we report it many times (up to filling seq buffer)
(Only the last socket of last slot)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>