RDMA/umem: Use correct value for SG entries in sg_copy_to_buffer()

With page combining, the assumption that number of SG entries in umem SGL
equal to number of system pages in umem no longer holds.

umem->sg_nents tracks the SG entries in umem SGL. Use it in
sg_pcopy_to_buffer() as opposed to ib_umem_num_pages(umem).

Fixes: d10bcf94 ("RDMA/umem: Combine contiguous PAGE_SIZE regions in SGEs")
Reported-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/umem.c

@@ -381,8 +381,8 @@ int ib_umem_copy_from(void *dst, struct ib_umem *umem, size_t offset,
 		return -EINVAL;
 	}
 
-	ret = sg_pcopy_to_buffer(umem->sg_head.sgl, ib_umem_num_pages(umem),
-				 dst, length, offset + ib_umem_offset(umem));
+	ret = sg_pcopy_to_buffer(umem->sg_head.sgl, umem->sg_nents, dst, length,
+				 offset + ib_umem_offset(umem));
 
 	if (ret < 0)
 		return ret;