md/raid10: add reshape support

A 'near' or 'offset' lay RAID10 array can be reshaped to a different
'near' or 'offset' layout, a different chunk size, and a different
number of devices.
However the number of copies cannot change.

Unlike RAID5/6, we do not support having user-space backup data that
is being relocated during a 'critical section'.  Rather, the
data_offset of each device must change so that when writing any block
to a new location, it will not over-write any data that is still
'live'.

This means that RAID10 reshape is not supportable on v0.90 metadata.

The different between the old data_offset and the new_offset must be
at least the larger of the chunksize multiplied by offset copies of
each of the old and new layout. (for 'near' mode, offset_copies == 1).

A larger difference of around 64M seems useful for in-place reshapes
as more data can be moved between metadata updates.
Very large differences (e.g. 512M) seem to slow the process down due
to lots of long seeks (on oldish consumer graded devices at least).

Metadata needs to be updated whenever the place we are about to write
to is considered - by the current metadata - to still contain data in
the old layout.

[unbalanced locking fix from Dan Carpenter <dan.carpenter@oracle.com>]
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.h

@@ -14,6 +14,7 @@ struct mirror_info {
 struct r10conf {
 	struct mddev		*mddev;
 	struct mirror_info	*mirrors;
+	struct mirror_info	*mirrors_new, *mirrors_old;
 	spinlock_t		device_lock;
 
 	/* geometry */
@@ -42,6 +43,9 @@ struct r10conf {
 	sector_t		dev_sectors;  /* temp copy of
 					       * mddev->dev_sectors */
 	sector_t		reshape_progress;
+	sector_t		reshape_safe;
+	unsigned long		reshape_checkpoint;
+	sector_t		offset_diff;
 
 	struct list_head	retry_list;
 	/* queue pending writes and submit them on unplug */
@@ -138,6 +142,7 @@ enum r10bio_state {
 	R10BIO_Uptodate,
 	R10BIO_IsSync,
 	R10BIO_IsRecover,
+	R10BIO_IsReshape,
 	R10BIO_Degraded,
 /* Set ReadError on bios that experience a read error
  * so that raid10d knows what to do with them.