f2fs: introduce FI_COMPRESS_RELEASED instead of using IMMUTABLE bit

Once we release compressed blocks, we used to set IMMUTABLE bit. But it turned
out it disallows every fs operations which we don't need for compression.

Let's just prevent writing data only.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

include/linux/f2fs_fs.h

@@ -229,6 +229,7 @@ struct f2fs_extent {
 #define F2FS_INLINE_DOTS	0x10	/* file having implicit dot dentries */
 #define F2FS_EXTRA_ATTR		0x20	/* file having extra attribute */
 #define F2FS_PIN_FILE		0x40	/* file should not be gced */
+#define F2FS_COMPRESS_RELEASED	0x80	/* file released compressed blocks */
 
 struct f2fs_inode {
 	__le16 i_mode;			/* file mode */