RDMA/rw: switch to dma_map_sgtable()

There are a couple of subtle error path bugs related to mapping the sgls:

- In rdma_rw_ctx_init(), dma_unmap would be called with an sg that could
  have been incremented from the original call, as well as an nents that
  is the dma mapped entries not the original number of nents called when
  mapped.

- Similarly in rdma_rw_ctx_signature_init, both sg and prot_sg were
  unmapped with the incorrect number of nents.

To fix this, switch to the sgtable interface for mapping which
conveniently stores the original nents for unmapping. This will get
cleaned up further once the dma mapping interface supports P2PDMA and
pci_p2pdma_map_sg() can be removed.

Fixes: 0e353e34 ("IB/core: add RW API support for signature MRs")
Fixes: a060b562 ("IB/core: generic RDMA READ/WRITE API")
Link: https://lore.kernel.org/r/20211001213215.3761-1-logang@deltatee.com


Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>