[DECNET]: Don't clear memory twice.

When dn_neigh.c was converted from kmalloc to kzalloc in commit
0da974f4 it was missed that
dn_neigh_seq_open was actually clearing the allocation twice was
missed.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>