libthr pshared: correct a bug in allocation

[kib]

Summary

When __thr_pshared_offpage() is called for allocation, it must not use
the cached offpage for the key.  Instead, the cached offpage must be
unmapped and removed from the cache, if any.

It is legitimate for the user code to unmap the shared lock object without
destroying it, and then mapping something over the freed VA to carry
another shared lock.  In this case the cached offpage must be un-cached.

PR:     269277

[kib]

Add pshared_destroy() helper (a separate commit).