Fix accidental EINVAL in move_blocks

When move blocks is staging it requires an overlapping offline extent to
cover the entire region to move.

It performs the stage by modifying extents at a time.  If there are
fragmented source extents it will modify each of them at a time in the
region.

When looking for the extent to match the source extent it looked from
the iblock of the start of the whole operation, not the start of the
source extent it's matching.  This meant that it would find a the first
previous online extent it just modified, which wouldn't be online, and
would return -EINVAL.

The fix is to have it search from the logical start of the extent it's
trying to match, not the start of the region.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/data.c

@@ -1285,7 +1285,7 @@ int scoutfs_data_move_blocks(struct inode *from, u64 from_off,
 
 			if (is_stage) {
 				ret = scoutfs_ext_next(sb, &data_ext_ops, &to_args,
-						       to_iblock, 1, &off_ext);
+						       to_start, 1, &off_ext);
 				if (ret)
 					break;