target: Add netlink command reply supported option for each device

Currently netlink command reply support option
(TCMU_ATTR_SUPP_KERN_CMD_REPLY) can be enabled only on module
scope. Because of that, once an application enables the netlink
command reply support, all applications using target_core_user.ko
would be expected to support the netlink reply. To make matters worse,
users will not be able to add a device via configfs manually.

To fix these issues, this patch adds an option to make netlink command
reply disabled on each device through configfs. Original
TCMU_ATTR_SUPP_KERN_CMD_REPLY is still enabled on module scope to keep
backward-compatibility and used by default, however once users set
nl_reply_supported=<NAGATIVE_VALUE> via configfs for a particular
device, the device disables the netlink command reply support.

Signed-off-by: Kenjiro Nakayama <nakayamakenjiro@gmail.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>