rds: stop using dmapool

RDMA ULPs should only perform DMA through the ib_dma_* API instead of
using the hidden dma_device directly.  In addition using the dma coherent
API family that dmapool is a part of can be very ineffcient on plaforms
that are not DMA coherent.  Switch to use slab allocations and the
ib_dma_* APIs instead.

Link: https://lore.kernel.org/r/20201106181941.1878556-6-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>