In the merge commit f9807aad
there were some omissions or errors.

ibuf_remove_free_page(): Return an error if the free list is corrupted
when removing the change buffer on an upgrade. A special 11.0 version of
commit 263932d5 would have been useful.

buf_page_get_gen(): Correctly handle the case that a page was being
concurrently read into the buffer pool and found out to be corrupted.
This was part of commit a4cda66e
but had been discarded in the merge.

Because MariaDB Server 11.0 has reached its end of life as of
commit 466ae1cf this fix is being applied
to the 11.1 branch.