net/rds: Use DMA memory pool allocation for rds_header

Currently, RDS calls ib_dma_alloc_coherent() to allocate a large piece
of contiguous DMA coherent memory to store struct rds_header for
sending/receiving packets.  The memory allocated is then partitioned
into struct rds_header.  This is not necessary and can be costly at
times when memory is fragmented.  Instead, RDS should use the DMA
memory pool interface to handle this.  The DMA addresses of the pre-
allocated headers are stored in an array.  At send/receive ring
initialization and refill time, this arrary is de-referenced to get
the DMA addresses.  This array is not accessed at send/receive packet
processing.

Suggested-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>