f2fs: compress: fix to disallow enabling compress on non-empty file

Compressed inode and normal inode has different layout, so we should
disallow enabling compress on non-empty file to avoid race condition
during inode .i_addr array parsing and updating.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: Fix missing condition]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>