scsi: tcmu: refactor nl wr_cache attr with new helpers

use new netlink events helpers tcmu_netlink_init() and
tcmu_netlink_send() to refactor netlink event attribute
TCMU_ATTR_WRITECACHE(belongs to TCMU_CMD_RECONFIG_DEVICE) which is also
emulate_write_cache in configFS.

Removed tcmu_netlink_event() since we have new netlink
events helpers now.

Signed-off-by: Zhu Lingshan <lszhu@suse.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>