net_namespace: trivial cleanup

Do not initialize net_kill_list twice.
list_replace_init() already takes care of initializing net_kill_list.
We don't need to initialize it with LIST_HEAD() beforehand.

Signed-off-by: xiao jin <jin.xiao@intel.com>
Reviewed-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>