When reshaping existing stripes, we should keep them on the same target
that they were allocated on; to do this, we need to add a field to the
btree stripe type.

This is a tad awkward, because we only have 8 bits left, and targets are
16 bits - but we only need to store a label, not a full target.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>