f2fs: remove unnecessary initialization

`flags' is used to save value from userspace, there is no need to
initialize it, and FS_FL_USER_VISIBLE is the mask for getflags.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>