target/rd: Convert to external rd_mcp_backend_dev_attrs

This patch converts RAMDISK to use an external set of device attributes,
and utilizes target_core_backend_configfs.h macros to generate a default
set of configfs extended-attr handlers.

It calls target_core_setup_sub_cits() to setup the initial config_item_type
based on existing target_core_configfs.c defaults, and using configfs_attribute
generated by DEF_TB_DEFAULT_ATTRIBS(rc_mcp) populates rd_mcp_backend_dev_attrs[]

It introduces no functional change for existing RD_MCP device attributes.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>