exfat: add support ioctl and FITRIM function

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>