Avoid unnecessary transform operations during prefetch

[JkshirsagarMaxlinear]

This change will prevent prefetch to perform unnecessary transform operations on ARC buffer.

Motivation and Context

In current ZFS, the read performance with prefetch enabled was getting affected because of the extra and unnecessary operations happening on ARC buffer. While the flag to avoid the ARC buffer was set in prefetch path, it was not getting assigned when actual zio is getting data from Disk to ARC.
This change will reduce the CPU utilization and boost the performance.
Closes #17008

Description

This change is mainly focused on utilizing the existing flag to avoid the unnecessary operations (buffer allocation and transform operations) in prefetch.

How Has This Been Tested?

This change was tested on following system:

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	23.10
Kernel Version	6.5.0-44-generic
Architecture	Intel(R) Xeon(R) Gold 6342 CPU
OpenZFS Version	2.2.6

Parameter	Info
Benchmarking Tool	IOR
Zpool Drives	6 NVME 1 TB Each
Zpool Configuration	RAID0
Zpool Recordsize	128k
ZFS Primarycache	all
ZFS compression	on (lz4)
ZFS encryption	off
ZFS checksum	on
ZFS zfs_compressed_arc_enabled	1

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

@JkshirsagarMaxlinear I took a liberty to edit the PR title. Also I suspect that too long first line of the commit message may not pass the style check. It is recommended to have the first line to be shorter to be more meaningful in one line commit lists. Also the commit message is somewhat confusing, since this change is not specific to compression. It in general saves on buffer allocation and filling, that cost something even if the block is not compressed. Please amend and force-push to make it nice. Please read the documents referenced in the PR checklist if not yet.

[JkshirsagarMaxlinear]

@amotin Thank you. I have updated the commit and pull request.

[JkshirsagarMaxlinear]

Thank you @amotin. It says that I am not authorized to merge the pull request.

[amotin]

@JkshirsagarMaxlinear Only few people can merge here. I'll do it after CI tests pass.

[amotin]

@JkshirsagarMaxlinear But I see you added here unneeded merge commit. Please clean it up with rebase. There should be only one commit.

[JkshirsagarMaxlinear]

Hi @amotin
I cleaned the commit and PR. Can you please merge if everything seems fine.