Clear PG_writeback for sync I/O error case

Commit 2b2861362f7dd09cc3167df8fddb6e2cb475018a accidentally
introduced this issue by only conditionally registering the
commit callback in the async case.

The error handing code for the dmu_tx_assign() failure case
relied on there always being a registered commit callback to
clear the PG_writeback bit. Since that is no longer strictly
true for the synchronous case we must explicitly invoke the
callback.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #961