Remove double counting HDR_L2ONLY_SIZE

Commit d962d5d didn't quite properly resolve the HDR_L2ONLY_SIZE
accounting. Accounting is now performed only in the constructor
and destructor which is a nice simplification. It should have
been removed the from create and destroy functions. This brings
up back in sync with upstream.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3533