RDMA/umem: Use ib_dma_max_seg_size instead of dma_get_max_seg_size

RDMA ULPs must not call DMA mapping APIs directly but instead use the
ib_dma_* wrappers.

Fixes: 0c16d963 ("RDMA/umem: Move to allocate SG table from pages")
Link: https://lore.kernel.org/r/20201106181941.1878556-3-hch@lst.de


Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>