f2fs: use the F2FS specific flags in f2fs_ioctl()

In f2fs_ioctl() function, it is using generic flags.
Since F2FS specific flags are defined. So lets use
those flags.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Pankaj Kumar <pankaj.km@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>