reader_concurrency_semaphore: background reads can trigger assert failure when the reader is made inactive

[denesb]

Since aab5954, readers request memory for I/O via request_memory() which returns a future<>. If memory usage is above the serialize limit, all such requests, except those that belong to the one blessed permit, will be queued for memory. This can be triggered from any I/O, including any background I/O the read is doing and when queued, the permit has its state changed to waiting_for_memory.
Such a read can be registered as inactive and currently this triggers an assert failure:

#6  0x0000000003e544f7 in reader_permit::impl::on_register_as_inactive (this=0x600005b1b200) at reader_concurrency_semaphore.cc:241
241             assert(_state == reader_permit::state::active_unused || _state == reader_permit::state::active_used);

[avikivity]

No vulnerable branches, not backporting.