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
9bf9055e ("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>