trx_undo_mem_create_at_db_start(): Invoke recv_sys_t::recover()
instead of buf_page_get_gen(), so that all undo log pages will be
recovered correctly. Failure to do this could prevent InnoDB from
starting up due to "Data structure corruption", or it could
potentially lead to a situation where InnoDB starts up but some
transactions were recovered incorrectly.

recv_sys_t::recover(): Only acquire a buffer-fix on the pages,
not a shared latch. This is adequate protection, because this function
is only being invoked during early startup when no "users" are modifying
buffer pool pages. The only writes are due to server bootstrap
(the data files being created) or crash recovery (changes from
ib_logfile0 being applied).

buf_page_get_gen(): Assert that the function is not invoked while crash
recovery is in progress, and that the special mode BUF_GET_RECOVER is
only invoked during crash recovery or server bootstrap.

All this should really have been part of
commit 850d6173 (MDEV-32042).