Add FITRIM ioctl to enable discarding unused blocks while mounted. As current exFAT doesn't have generic ioctl handler, add empty ioctl function first, and add FITRIM handler. Signed-off-by: Hyeongseok Kim <hyeongseok@gmail.com> Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com> Acked-by: Sungjong Seo <sj1557.seo@samsung.com> Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>