RDS: memory allocated must be align to 8

Fix issue in 'rds_ib_cong_recv' when accessing unaligned memory
allocated by 'rds_page_remainder_alloc' using uint64_t pointer.

Signed-off-by: Shamir Rabinovitch <shamir.rabinovitch@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>