md: load/store badblock list from v1.x metadata

Space must have been allocated when array was created.
A feature flag is set when the badblock list is non-empty, to
ensure old kernels don't load and trust the whole device.

We only update the on-disk badblocklist when it has changed.
If the badblocklist (or other metadata) is stored on a bad block, we
don't cope very well.

If metadata has no room for bad block, flag bad-blocks as disabled,
and do the same for 0.90 metadata.

Signed-off-by: NeilBrown <neilb@suse.de>

include/linux/raid/md_p.h

@@ -245,10 +245,16 @@ struct mdp_superblock_1 {
 	__u8	device_uuid[16]; /* user-space setable, ignored by kernel */
 	__u8	devflags;	/* per-device flags.  Only one defined...*/
 #define	WriteMostly1	1	/* mask for writemostly flag in above */
-	__u8	pad2[64-57];	/* set to 0 when writing */
+	/* Bad block log.  If there are any bad blocks the feature flag is set.
+	 * If offset and size are non-zero, that space is reserved and available
+	 */
+	__u8	bblog_shift;	/* shift from sectors to block size */
+	__le16	bblog_size;	/* number of sectors reserved for list */
+	__le32	bblog_offset;	/* sector offset from superblock to bblog,
+				 * signed - not unsigned */
 
 	/* array state information - 64 bytes */
-	__le64	utime;		/* 40 bits second, 24 btes microseconds */
+	__le64	utime;		/* 40 bits second, 24 bits microseconds */
 	__le64	events;		/* incremented when superblock updated */
 	__le64	resync_offset;	/* data before this offset (from data_offset) known to be in sync */
 	__le32	sb_csum;	/* checksum up to devs[max_dev] */
@@ -270,8 +276,8 @@ struct mdp_superblock_1 {
 					   * must be honoured
 					   */
 #define	MD_FEATURE_RESHAPE_ACTIVE	4
+#define	MD_FEATURE_BAD_BLOCKS		8 /* badblock list is not empty */
 
-#define	MD_FEATURE_ALL			(1|2|4)
+#define	MD_FEATURE_ALL			(1|2|4|8)
 
 #endif 
-