vgsplit: Fix intermediate metadata corruption.

vgsplit: Fix intermediate metadata corruption.

Changing the VG of a PV uses the same on-disk mechanism as vgrename.
This relies on recognising both the old and new VG names.  Prior to this
patch the vgsplit code incorrectly provided the new VG name twice
instead of the old and new ones.  This lead the low-level mechanism not
to recognise the device as already belonging to a VG and so paying no
attention to the location of its existing metadata, sometimes partly
overwriting it and then later trying to read the corrupt metadata and
issuing a checksum error.