IB/iser: Keep IB device attributes under iser_device

For T10-PI offload support, we will need to know the device signature
offload capability upon every connection establishment.

This patch does not change any functionality.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Alex Tabachnik <alext@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>