rds: rds_message_zcopy_from_user() can be static

Fixes: d40a126b ("rds: refactor zcopy code into rds_message_zcopy_from_user")
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>