in_head: switched the type of buf_size to match the config_map

[rightblank]

fix cannot allocate memory on big endian system,

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[leonardo-albertovich]

I changed the subject to properly represent the change made. If you want to make a note about this issue showing on s390 in the description that's ok but this is not an endianness or s390 specific issue and we should clearly communicate the nature of it so someone who is not well informed does not misinterpret it.