RDMA/rxe: Use correct sizing on buffers holding page DMA addresses

The buffer that holds the page DMA addresses is sized off umem->nmap.
This can potentially cause out of bound accesses on the PBL array when
iterating the umem DMA-mapped SGL. This is because if umem pages are
combined, umem->nmap can be much lower than the number of system pages
in umem.

Use ib_umem_num_pages() to size this buffer.

Cc: Moni Shoua <monis@mellanox.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/sw/rxe/rxe_mr.c

@@ -179,7 +179,7 @@ int rxe_mem_init_user(struct rxe_pd *pd, u64 start,
 	}
 
 	mem->umem = umem;
-	num_buf = umem->nmap;
+	num_buf = ib_umem_num_pages(umem);
 
 	rxe_mem_init(access, mem);