Allow non-orphan PVs with two metadata areas to be resized.

Allow non-orphan PVs with two metadata areas to be resized.

We allow writing non-orphan PVs only for resize now. The "orphan PV" assert
in pv_write fn uses the "allow_non_orphan" parameter to control this assert.
However, we should find a more elaborate solution so we can remove this
restriction altogether (pv_write together with vg_write is not atomic, we
need to find a safe mechanism so there's an easy revert possible in case of
an error).