FreeBSD zio_crypt.c: initialize uio variables before access

In zio_crypt_key_wrap and zio_crypt_key_unwrap, the cuio_s variable was
not initialized before the calls to zfs_uio_init, leading to
uninitialized access to cuio_s.uio_offset. Initialize it to avoid gcc
warnings.

Similar issue as fixed in 2bf152021 ("Fix gcc uninitialized warning in
FreeBSD zio_crypt.c")

Signed-off-by: Ryan Libby <rlibby@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #17863