[NETNS]: Consolidate hashes creation in netdev_init()

The dev_name_hash and the dev_index_hash are now booth kmalloc-ed
(and each element is properly initialized as usually) so I think
it's worth consolidating this code making it look nicer (and
saving 28 bytes of .text section ;) )

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>