[ATM]: Simplify /proc/net/atm/arp opening

The iterator state->ns.neigh_sub_iter initialization is moved from
arp_seq_open to clip_seq_start for convinience. This should not be a
problem as the iterator will be used only after the seq_start
callback.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>