MDEV-35525: Index corruption in reverse scans

[dr-m]

• The Jira issue number for this PR is: MDEV-35525

Description

btr_cur_t::search_leaf(): In the BTR_SEARCH_PREV and BTR_MODIFY_PREV modes, reset the previous search status before invoking page_cur_search_with_match(). Otherwise, we the search could invoke in a totally wrong subtree.

Release Notes

See MDEV-35525.

How can this PR be tested?

The bug was reproduced as part of testing #3562 in the random query generator (RQG) by using innodb_change_buffering=all and innodb_page_size=4k. Possibly also using innodb_change_buffering_debug.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

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