net/rds: Fix new sparse warning

c0adf54a introduced new sparse warnings:
  CHECK   /home/dahern/kernels/linux.git/net/rds/ib_cm.c
net/rds/ib_cm.c:191:34: warning: incorrect type in initializer (different base types)
net/rds/ib_cm.c:191:34:    expected unsigned long long [unsigned] [usertype] dp_ack_seq
net/rds/ib_cm.c:191:34:    got restricted __be64 <noident>
net/rds/ib_cm.c:194:51: warning: cast to restricted __be64

The temporary variable for sequence number should have been declared as __be64
rather than u64. Make it so.

Signed-off-by: David Ahern <david.ahern@oracle.com>
Cc: shamir rabinovitch <shamir.rabinovitch@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>