RDMA/umem: Don't check for a negative return value of dma_map_sg_attrs()

dma_map_sg_attrs() returns 0 on error and can't return a negative number
(ensured by BUG_ON), so don't check.
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/umem.c

@@ -206,7 +206,7 @@ struct ib_umem *ib_umem_get(struct ib_ucontext *context, unsigned long addr,
 				  DMA_BIDIRECTIONAL,
 				  dma_attrs);
 
-	if (umem->nmap <= 0) {
+	if (!umem->nmap) {
 		ret = -ENOMEM;
 		goto out;
 	}