f2fs: make file immutable even if releasing zero compression block

When we use F2FS_IOC_RELEASE_COMPRESS_BLOCKS ioctl, if we can't find
any compressed blocks in the file even with large file size, the
ioctl just ends up without changing the file's status as immutable.
It makes the user, who expects that the file is immutable when it
returns successfully, confused.

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