This patch implements IB_WR_REG_SIG_MR posted by the user.

Baisically this WR involves 3 WQEs in order to prepare and properly
register the signature layout:

1. post UMR WR to register the sig_mr in one of two possible ways:
    * In case the user registered a single MR for data so the UMR data segment
      consists of:
      - single klm (data MR) passed by the user
      - BSF with signature attributes requested by the user.
    * In case the user registered 2 MRs, one for data and one for protection,
      the UMR consists of:
      - strided block format which includes data and protection MRs and
        their repetitive block format.
      - BSF with signature attributes requested by the user.

2. post SET_PSV in order to set the memory domain initial
   signature parameters passed by the user.
   SET_PSV is not signaled and solicited CQE.

3. post SET_PSV in order to set the wire domain initial
   signature parameters passed by the user.
   SET_PSV is not signaled and solicited CQE.

* After this compound WR we place a small fence for next WR to come.

This patch also introduces some helper functions to set the BSF correctly
and determining the signature format selectors.
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>