decnet: Don't leak entries when rebuilding zone.

As noticed by Ben Hutchings, when we move entries from
one table to another we leak all except the first entry.

Put back the "next" variable removed by commit
9bf9055eb716f85372c41b3fbc51f90bc7653740 ("decnet: Fix set-but-unused
variable.") and use it properly.

Reported-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>