Handle new dnode size in incremental backup stream

Handle new dnode size in incremental backup stream

When receiving an incremental backup stream, call
dmu_object_reclaim_dnsize() if an object's dnode size differs between
the incremental source and target. Otherwise it may appear that a
dnode which has shrunk is still occupying slots which are in fact
free. This will cause a failure to receive new objects that should
occupy the now-free slots.

Add a test case to verify that an incremental stream containing
objects with changed dnode sizes can be received without error. This
test case fails without this change.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #6366
Closes #6576