RDS: fix endianness for dp_ack_seq

dp->dp_ack_seq is used in big endian format. We need to do the
big endianness conversion when we assign a value in host format
to it.

Signed-off-by: Qing Huang <qing.huang@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>