pvmove: add missing segment merging

pvmove: add missing segment merging

When pvmove is finished and metadata are updated, the code missed
to merge possible mergable segments - so add explicit merging
call after pvmoved volumes are unlocked.

This avoids weird results where i.e. lvs could have been reporting
non-matching segments as lvs upon metadata read is doing silent segment
merging while dm table left after pvmove was still preserving
non-merged segments.