libthr pshared: correct a bug in allocation

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
Reported by: rau8344@gmail.com
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D38345