Fix module build with -Werror

This is a direct commit to zfs-2.1-release to fix release builds that
error out on an unused variable.  The issue is avoided on master by a
huge series of commits that change how the ASSERT macros work, but that
is not feasible to backport.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #13194 
Closes #13196

module/zfs/arc.c

@@ -1205,7 +1205,7 @@ static void
 hdr_l2only_dest(void *vbuf, void *unused)
 {
 	(void) unused;
-	arc_buf_hdr_t *hdr = vbuf;
+	arc_buf_hdr_t *hdr __maybe_unused = vbuf;
 
 	ASSERT(HDR_EMPTY(hdr));
 	arc_space_return(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);