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>

fs/f2fs/file.c

@@ -1821,6 +1821,8 @@ static int f2fs_setflags_common(struct inode *inode, u32 iflags, u32 mask)
 		if (iflags & F2FS_COMPR_FL) {
 			if (!f2fs_may_compress(inode))
 				return -EINVAL;
+			if (S_ISREG(inode->i_mode) && inode->i_size)
+				return -EINVAL;
 
 			set_compress_context(inode);
 		}