buf_page_get_gen(): After recv_sys_t::recover_low() returned,
the page must not be read-fixed, but it may be write-fixed,
because the io-fix state is protected by block->page.lock,
which we are not holding yet.

Also, let us copy the block descriptor state to a local variable
for examination, so that in case an assertion would fail again,
we will have the sampled state in the core dump. In a core dump of
the assertion failure, we had block->page.fix() == buf_page_t::UNFIXED,
that is, the assertion expression was holding again.