f2fs: add ioctl to expose current features

This patch adds an ioctl to provide feature information to user.
For exapmle, SQLite can use this ioctl to detect whether f2fs support atomic
write or not.

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