target: make the ->get_cdb method optional

The most commonly used file, iblock and rd backends have no use for
a per-task CDB and thus don't need a method to copy it into their
otherwise unused CDB fields.

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

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

diff --cc drivers/target/target_core_file.h
Simple merge

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

diff --cc drivers/target/target_core_iblock.h
Simple merge

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

diff --cc drivers/target/target_core_rd.h
Simple merge

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