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 openzfs#16688

module/os/freebsd/zfs/zio_crypt.c

@@ -1686,11 +1686,10 @@ zio_do_crypt_data(boolean_t encrypt, zio_crypt_key_t *key,
 	freebsd_crypt_session_t *tmpl = NULL;
 	uint8_t *authbuf = NULL;
 
-
+	memset(&puio_s, 0, sizeof (puio_s));
+	memset(&cuio_s, 0, sizeof (cuio_s));
 	zfs_uio_init(&puio, &puio_s);
 	zfs_uio_init(&cuio, &cuio_s);
-	memset(GET_UIO_STRUCT(&puio), 0, sizeof (struct uio));
-	memset(GET_UIO_STRUCT(&cuio), 0, sizeof (struct uio));
 
 #ifdef FCRYPTO_DEBUG
 	printf("%s(%s, %p, %p, %d, %p, %p, %u, %s, %p, %p, %p)\n",