target: Propigate backend read-only to core_tpg_add_lun

This patch adds a DF_READ_ONLY flag that is used by IBLOCK to
signal when a backend has been set to read-only mode, in order
to propigate read-only status up to core_tpg_add_lun() for all
future LUN fabric exports.

With this is place, existing emulation for reporting read-only
in spc_emulate_modesense() and normal transport_lookup_cmd_lun()
TCM_WRITE_PROTECTED status checking just works as expected.

Reported-by: Joeue Deng <joeue404@gmail.com>
Reported-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

include/target/target_core_base.h

@@ -730,6 +730,7 @@ struct se_device {
 #define DF_EMULATED_VPD_UNIT_SERIAL		0x00000004
 #define DF_USING_UDEV_PATH			0x00000008
 #define DF_USING_ALIAS				0x00000010
+#define DF_READ_ONLY				0x00000020
 	/* Physical device queue depth */
 	u32			queue_depth;
 	/* Used for SPC-2 reservations enforce of ISIDs */