IB/mlx5: Initialize mlx5_ib_qp signature-related members

If user requested signature enable we initialize relevant mlx5_ib_qp
members.  We mark the qp as sig_enable and we increase the effective
SQ size, but still limit the user max_send_wr to original size
computed.  We also allow the create_qp routine to accept sig_enable
create flag.

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