Fix issue in receive_object() during reallocation

Fix issue in receive_object() during reallocation

When receiving an object to a previously allocated interior slot
the new object should be "allocated" by setting DMU_NEW_OBJECT,
not "reallocated" with dnode_reallocate().  For resilience verify
the slot is free as required in case the stream is malformed.

Add a test case to generate more realistic incremental send streams
that force reallocation to occur during the receive.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8067
Closes #8614