[DECNET] ROUTE: remove unecessary alignment

Same alignment requirement was removed on IP route cache in the past.

This alignment actually has bad effect on 32 bit arches, uniprocessor,
since sizeof(dn_rt_hash_bucket) is forced to 8 bytes instead of 4.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>