RDMA/core: Set sgtable nents when using ib_dma_virt_map_sg()

RDMA/core: Set sgtable nents when using ib_dma_virt_map_sg()

ib_dma_map_sgtable_attrs() should be mapping the sgls and setting nents
but the ib_uses_virt_dma() path falls back to ib_dma_virt_map_sg() which
will not set the nents in the sgtable.

Check the return value (per the map_sg calling convention) and set
sgt->nents appropriately on success.

Fixes: 79fbd3e1241c ("RDMA: Use the sg_table directly and remove the opencoded version from umem")
Link: https://lore.kernel.org/r/20211013165942.89806-1-logang@deltatee.com
Reported-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Tested-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>