Once we destruct SharedLRU, SharedLRU::weak_refs map is destroyed.
As a weak refernce might outlive the SharedLRU itself, when destroying
the object via the custom Deleter, we try to access the already
destroyed SharedLRU instance's weak ref map.

Instead, invalidate the custom Deleter (Deleter::cache), when
destructing the SharedLRU.

Fixes: https://tracker.ceph.com/issues/66478

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