IPoIB: Use the right function to do DMA unmap pages

Pages that were mapped using ib_dma_map_page() should be unmapped
using ib_dma_unmap_page().

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Reviewed-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>