Add more constraints for block cloning

[amotin]

• We cannot clone into files with smaller block size if there is more than one block, since we can not grow the block size.
• Block size must be power-of-2 if destination offset != 0, since there can be no multiple blocks of non-power-of-2 size.

The first should handle the case when destination file has several blocks but still is not bigger than one block of the source file. The second fixes panic in dmu_buf_hold_array_by_dnode() on attempt to concatenate files with equal but non-power-of-2 block sizes.

While there, assert that error is reported if we made no progress.

How Has This Been Tested?

Reproduced kernel panic on FreeBSD with the script from https://lists.freebsd.org/archives/freebsd-current/2023-September/004610.html and confirmed that it works after.

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.

[behlendorf]

cc: @robn @oromenahar.

[amotin]

Another theoretical guess while thinking about this area: if we use block cloning to concatenate one 512 byte file to another 512 byte file we'll get a file with two 512 byte blocks, but I am not sure we really want that. We may wish to restrict minimal copy size to allow block sizes to grow towards recordsize in normal way instead.

[pjd]

Looks good to me. Thank you Alex for looking into this!