Add more constraints for block cloning.

• 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.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.