Some ZIO micro-optimizations.

• Pack struct zio_prop by 4 bytes from 84 to 80.
  • Skip new child ZIO locking while linking to parent. The newly

allocated ZIO is not externally visible yet, so nobody should care.

• Skip io_bp_copy writes when not used (write && non-debug).

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14985