mlx5_core: Fix signature handover operation for interleaved buffers

When the data and protection are interleaved in the memory domain, no
need to expand the mkey total length.

At the moment no Linux user works (iSER initiator & target) in
interleaved mode. This may change in the future as for SCSI
pass-through devices there is no real point in target performing
de-interleaving and re-interleaving of the protection data in the PT
stage. Regardless, signature verbs support this mode.

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