zil: Don't expect zio_shrink() to succeed.

At least for RAIDZ zio_shrink() does not reduce zio size, but reduced
wsz in that case likely results in writing uninitialized memory.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14853