module: zfs: arc: hdr_full_crypt_dest: drop unevaulated-only variable

This explodes as -Wunused-variable on GCC 8.5.0, despite it being used,
just not in an evaluated context

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes openzfs#13195

module/zfs/arc.c

@@ -1204,11 +1204,11 @@ hdr_full_dest(void *vbuf, void *unused)
 static void
 hdr_full_crypt_dest(void *vbuf, void *unused)
 {
-	(void) unused;
-	arc_buf_hdr_t *hdr = vbuf;
+	(void) vbuf, (void) unused;
 
 	hdr_full_dest(vbuf, unused);
-	arc_space_return(sizeof (hdr->b_crypt_hdr), ARC_SPACE_HDRS);
+	arc_space_return(sizeof (((arc_buf_hdr_t *)NULL)->b_crypt_hdr),
+	    ARC_SPACE_HDRS);
 }
 
 static void