f2fs: add ioctl to flush data from faster device to cold area

This patch adds an ioctl to flush data in faster device to cold area. User can
give device number and number of segments to move. It doesn't move it if there
is only one device.

The parameter looks like:

struct f2fs_flush_device {
	u32 dev_num;		/* device number to flush */
	u32 segments;		/* # of segments to flush */
};

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>