IB/mlx5: Use extended internal signature layout

Rather than using the basic BSF layout which utilizes a pre-configured
signature settings (sufficient for current DIF implementation), we use
the extended BSF layout to expose advanced signature settings. These
settings will also be exposed to the user later.

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