Linux: Fix uninitialized variable usage in zio_do_crypt_data()

Coverity complained about this. An error from hkdf_sha512() before uio
initialization will cause pointers to uninitialized memory to be passed
to zio_crypt_destroy_uio(). This is a regression that was introduced
by cf63739191b6cac629d053930a4aea592bca3819. Interestingly, this never
affected FreeBSD, since the FreeBSD version never had that patch ported.
Since moving uio initialization to the top of this function would slow
down the qat_crypt() path, we only move the memset() calls to the top
of the function. This is sufficient to fix this problem.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13944