Mark TX_COMMIT transaction with TXG_NOTHROTTLE.

TX_COMMIT has no on-disk representation and does not produce any more
dirty data. It should not wait for anything, and even just skipping
the checks if not waiting gives improvement noticeable in profiler.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14798