target/sbc: Add sbc_dif_generate software emulation

This patch adds WRITE_INSERT emulation within target-core
using TYPE1 / TYPE3 PI modes in sbc_dif_generate() code.

This is useful in order for existing legacy fabrics that do not
support protection offloads to interact with backend devices that
currently have T10 PI enabled.

v2 changes:
   - Rename to sbc_dif_generate() (Sagi)

Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Quinn Tran <quinn.tran@qlogic.com>
Cc: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>