Target/dif: Introduce protection-passthough-only mode

Some transports (iSCSI/iSER/SRP/FC) support hardware INSERT/STRIP
capabilities while other transports like loopback/vhost-scsi need
perform this is software.

This patch allows fabrics using SCF_PASSTHROUGH_SG_TO_MEM_NOALLOC
to signal the early LUN scan handling case where PROTECT CDB bits
are set, but no fabric buffer has been provided.

For transports which use generic new command these buffers have yet
to be allocated.

Also this way, target may support protection information
against legacy initiators (writes are inserted and reads
are stripped).

(Only set prot_pto for loopback during early special case - nab)

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