target: add a parse_cdb method to the backend drivers

Instead of trying to handle all SCSI command sets in one function
(transport_generic_cmd_sequencer) call out to the backend driver to perform
this functionality.  For pSCSI a copy of the existing code is used, but for
all virtual backends we can use a new parse_sbc_cdb helper is used to
provide a simple SBC emulation.

For now this setups means a fair amount of duplication between pSCSI and the
SBC library, but patches later in this series will sort out that problem.

(nab: Fix up build failure in target_core_pscsi.c)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/drivers/target/Makefile b/drivers/target/Makefile
index 70cab2a..50b887b 100644
--- a/drivers/target/Makefile
+++ b/drivers/target/Makefile
@@ -10,6 +10,7 @@
 				   target_core_tpg.o \
 				   target_core_transport.o \
 				   target_core_cdb.o \
+				   target_core_sbc.o \
 				   target_core_spc.o \
 				   target_core_ua.o \
 				   target_core_rd.o \