f2fs: make gc_urgent and gc_segment_mode sysfs node readable

Changed a way of showing values of them to use strings.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>