pvchange: fix file locking deadlock

pvchange: fix file locking deadlock

Calling clear_hint_file() to invalidate hints would acquire
the hints flock before the global flock which could cause deadlock.
The lock order requires the global lock to be taken first.

pvchange was always invalidating hints, which was unnecessary;
only invalidate hints when changing a PV uuid.  Because of the
lock ordering, take the global lock before clear_hint_file which
locks the hints file.