IB/core: fix unmap_sg argument

__ib_umem_release calls dma_unmap_sg with a different number of
sg_entries than ib_umem_get uses for dma_map_sg. This might cause
trouble for implementations that merge sglist entries and results
in the following dma debug complaint:

DMA-API: device driver frees DMA sg list with different entry
         count [map count=2] [unmap count=1]

Fix it by using the correct value.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>