FreeBSD: Code cleanup in zio_crypt

Address some unused value and control flow issues flagged by Coverity.

Unreachable code is pruned and unused values are avoided.
Some scattered sections are reordered for coherence.

We can assume kmem_alloc(n, KM_SLEEP) doesn't fail, so there is no need
to check if it returned NULL. The allocated memory doesn't need to be
zeroed, other than the last iovec (the MAC).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10884