False positives for buf_page_t::ibuf_exist are acceptable,
because it does not hurt to unnecessarily invoke
ibuf_merge_or_delete_for_page().

Invoking buf_page_get_gen() in a read completion function
is a definite no-no, because it could trigger a page flush
or cause the server to run out of buffer pool.

With some MDEV-23855 changes present, the test innodb.purge_secondary
occasionally failed due to the table having been dropped while
ibuf_page_exists() invoked buf_page_get_gen().

Reviewed by: Thirunarayanan Balathandayuthapani