IB/iser: Fix dma_nents type definition

The retured value from ib_dma_map_sg saved in dma_nents variable. To avoid
future mismatch between types, define dma_nents as an integer instead of
unsigned.

Fixes: 57b26497 ("IB/iser: Pass the correct number of entries for dma mapped SGL")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>