Add kstats tracking gang allocations

[pcd1193182]

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

Motivation and Context

Gang blocks have a significant impact on the long and short term performance of a zpool, but there is not a lot of observability into whether they're being used.

Description

This change adds gang-specific kstats to ZFS, to better allow users to see whether ganging is happening. There are only two kstats in this version; number of gang fallbacks, and number of multi-level gang fallbacks. If people have suggestions for other stats, they can be added.

How Has This Been Tested?

Forced ganging on for small block sizes, checked that stats increased appropriately.

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]

Few thoughts to think:

• Names like ziostat.normal_fallbacks are not exactly hint that it is about allocation.
• We also have another allocation path for ZIL.
• And I see we have a few more allocation-related counters inside metaslab.
• There are some style errors.

[amotin]

@pcd1193182 "error: commit message body contains line over 72 characters"