IB/srp: Initialize dma_length in srp_map_idb

Without this sg_dma_len will return 0 on architectures tha have
the dma_length field.

Fixes: commit f7f7aab1 ("IB/srp: Convert to new registration API")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>