btrfs: update stripe_extent delete loop assumptions

btrfs_delete_raid_extent() was written under the assumption, that it's
call-chain always passes a start, length tuple that matches a single
extent. But btrfs_delete_raid_extent() is called by
do_free_extent_accounting() which in turn is called by
__btrfs_free_extent().

But this call-chain passes in a start address and a length that can
possibly match multiple on-disk extents.

To make this possible, we have to adjust the start and length of each
btree node lookup, to not delete beyond the requested range.

Reviewed-by: Qu Wenruo <[e-mail]>
Signed-off-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/raid-stripe-tree.c

···
 		if (ret)
 			break;
 
+		start += key.offset;
+		length -= key.offset;
+		if (length == 0)
+			break;
+
 		btrfs_release_path(path);
 	}