net: netlink: make netlink_walk_start() void return type

netlink_walk_start() needed to return an error code because of
rhashtable_walk_init(). but that was converted to rhashtable_walk_enter()
and it is a void type function. so now netlink_walk_start() doesn't need
any return value.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>