fix calling to ib_dma_unmap_sg

The ib_dma_unmap_sg (like sg_unmap_sg) should receive the numebr of
the input nents to ib_dma_map_sg and not the number of returned from
the mapping API. Fix that.

Change-Id: Idddfa85070e320935e6abaec7056d0e51899d51e
Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>