IB/isert: Initialize T10-PI resources

Introduce pi_context to hold relevant RDMA protection resources.
We eliminate data_key_valid boolean and replace it with indicators
container to indicate:
- Is the descriptor protected (registered via signature MR)
- Is the data_mr key valid (can spare LOCAL_INV WR)
- Is the prot_mr key valid (can spare LOCAL_INV WR)
- Is the sig_mr key valid (can spare LOCAL_INV WR)

Upon connection establishment check if network portal is T10-PI
enabled and allocate T10-PI resources if necessary, allocate
signature enabled memory regions and mark connection queue-pair
as signature enabled.

(Fix context change for v3.14-rc6 code - nab)

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>