txg_sync should handle write errors in ZIL

The txg_sync thread will see certain buffers in a DR_IN_DMU_SYNC state
when ZIL is writing them out. Then it waits until the state changes, but
has an assertion to check that they were not DR_NOT_OVERRIDDEN. If the
data write failed with an error, ZIL will put it into the
DR_NOT_OVERRIDDEN state. It looks like the code will handle that state
without an issue, so we can just delete the assertion.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@klarasystems.com>
Sponsored-By: Wasabi Technology, Inc.
Closes #14283