f2fs: expose features to sysfs entry

This patch exposes what features are supported by current f2fs build to sysfs
entry via:

/sys/fs/f2fs/features/
/sys/fs/f2fs/dev/features

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>