rhashtable: initialize all rhashtable walker members

Commit f2dba9c6 ("rhashtable: Introduce rhashtable_walk_*") forgot to
initialize the members of struct rhashtable_walker after allocating it, which
caused an undefined value for 'resize' which is used later on.

Fixes: f2dba9c6

 ("rhashtable: Introduce rhashtable_walk_*")
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>