Fix metaslab group fragmentation math

[amotin]

Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation.

While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG.

How Has This Been Tested?

Sequentially filled half of metaslabs to almost full, leaving half almost empty, and observed incorrect 8% fragmentation. Applied the patch and saw the proper 0%. Randomly overwritten the data and observed that fragmentation shown actual 10% due to plenty of small holes.

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.