[krbdev.mit.edu #1312] profile locking at wrong level when re-reading file

[Ken Raeburn via RT]

With the profile-sharing code enabled, the mutex protects the linked
list of prf_data_t objects and their refcounts.  However, once one of
these objects has been acquired for a profile instance, it's used
without locking.  Mostly it's read-only access, but if the on-disk file
has changed, the tree of nodes will be freed and rebuilt, with no mutex
protection.