f2fs/009: use common _scratch_shutdown helper

f2fs has supported ioctl(_IOR ('X', 125, uint32_t)), so that
"f2fs_io shutdown" can be completely replaced w/ _scratch_shutdown(),
let's clean up in f2fs/009.

Suggested-by: Zorro Lang <zlang@kernel.org>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Zorro Lang <zlang@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>