Fix gcc uninitialized warning in FreeBSD zio_crypt.c

In FreeBSD's zio_do_crypt_data(), ensure that two struct uio
variables are cleared before copying data out of them. This avoids
accessing garbage data, and fixes gcc -Wuninitialized warnings.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Toomas Soome <tsoome@me.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Dimitry Andric <dimitry@andric.com>
Closes #16688