target: make the tpg_get_default_depth method optional

All fabric drivers except for iSCSI always return 1, so implement
that as default behavior.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc Documentation/target/tcm_mod_builder.py
Simple merge

diff --cc drivers/infiniband/ulp/srpt/ib_srpt.c
Simple merge

diff --cc drivers/scsi/qla2xxx/tcm_qla2xxx.c
Simple merge

diff --cc drivers/target/loopback/tcm_loop.c
Simple merge

diff --cc drivers/target/sbp/sbp_target.c
Simple merge

diff --cc drivers/target/target_core_configfs.c
Simple merge

diff --cc drivers/target/target_core_tpg.c
Simple merge

diff --cc drivers/target/tcm_fc/tfc_conf.c
Simple merge

diff --cc drivers/usb/gadget/legacy/tcm_usb_gadget.c
Simple merge

diff --cc drivers/vhost/scsi.c
Simple merge

diff --cc drivers/xen/xen-scsiback.c
Simple merge