backport enforce arc_dnode_limit to 2.2

[siv0]

Motivation and Context

This patch is a backport of a7a144e ("enforce arc_dnode_limit")
a7a144e
for the 2.2 branch.

According to a simple reproducer of a colleague of mine the patch causes the dnode cache to shrink again after growing, while it remains at a high level without it. See the discussion (with details on the reproducer) at:
https://lore.proxmox.com/pve-devel/[email protected]/
for details on the reproducer.

Description

Started with a cherry-pick of a7a144e65 and adapted the code to changes from
55427ad ("Several improvements to ARC shrinking (#16197)")
5b9f3b7 ("Soften pruning threshold on not evictable metadata")
which are present in 2.3.3, but not in 2.2.8
I'm not familiar with that part of the code - so the changes might be missing something.

How Has This Been Tested?

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.
• [x ] I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[siv0]

I think the failing checks on debian11/bullseye should be addressed by:
#17569 (CI: Remove Debian backports #17569)

[lzsaver]

For the most part, it looks like my own backport that I used before this PR. I hope someone can do a review.