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This reverts part of commit 212994f7
(MDEV-28974) and implements a better fix that works in that special case
while avoiding other failures.

fil_name_process(): Do not rename the tablespace in deferred_spaces;
it already contains the latest name for the space id.

deferred_spaces::create(): In mariadb-backup --prepare,
replace absolute data directory file path with short name
relative to the backup directory and store it as filename while
deferring tablespace file creation.