ZIL: Refactor TX_WRITE encryption similar to TX_CLONE_RANGE

It should be purely textual change to make the code more readable.
Should cause no functional difference.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Tom Caputi <caputit1@tcnj.edu>
Reviewed-by: Sean Eric Fagan <sef@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Edmund Nadolski <edmund.nadolski@ixsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15543
Closes #15513