mlx5: Fix mlx5_ib_map_mr_sg mr length

In case we got an initial sg_offset, we need to
account for it in the mr length.

Cc: stable@vger.kernel.org
Fixes: ff2ba993

 ("IB/core: Add passing an offset into the SG to
ib_map_mr_sg")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>