metaslab: tuneable to better control force ganging

[robn]

Motivation and Context

metaslab_force_ganging isn't enough to actually force ganging, because it still only forces 3% of the time. This adds metaslab_force_ganging_pct so we can configure how often to force ganging.

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

How Has This Been Tested?

Heavily as part of some other debugging work, but here's a simple standalone run:

# echo 32768 > /sys/module/zfs/parameters/metaslab_force_ganging
# echo 100 > /sys/module/zfs/parameters/metaslab_force_ganging_pct
# zpool create tank loop0

# dd if=/dev/random of=/tank/file bs=128K count=4
4+0 records in
4+0 records out
524288 bytes (524 kB, 512 KiB) copied, 0.0018605 s, 282 MB/s
# zpool sync
# grep 'trying ganging' /proc/spl/kstat/zfs/dbgmsg | wc -l
16
# zdb -b tank 2>/dev/null | grep ganged
  ganged count:                 8

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.