MDEV-26740 Inplace alter rebuild increases file size

[Thirunarayanan]

• The Jira issue number for this PR is: MDEV-26740

Description

PageBulk::init(): Unnecessary reserves the extent before allocating a page for bulk insert. btr_page_alloc() capable of handing the extending of tablespace.

Basing the PR against the correct MariaDB version

• This is a new feature and the PR is based against the latest MariaDB development 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.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[dr-m]

Very good. I see that this is covered by the existing test innodb_zip.innochecksum_3, so we do not necessarily need a new test for this.