IB/umem: Fix use of npages/nmap fields

In ib_umem structure npages holds original number of sg entries, while
nmap is number of DMA blocks returned by dma_map_sg.

Fixes: c5d76f13 ('IB/core: Add umem function to read data from user-space')
Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>