Linux: Fix detection of register_sysctl_sz

Adjust the m4 function to mimic sentinel we use in spl-proc.c
This fixes the detection on kernels compiled with CONFIG_RANDSTRUCT=y

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Pavel Snajdr <snajpa@snajpa.net>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ivan Volosyuk <Ivan.Volosyuk@gmail.com>
Closes: openzfs#16620
Closes: openzfs#16805

config/kernel-register_sysctl_table.m4

@@ -36,7 +36,7 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_REGISTER_SYSCTL_SZ], [
 	ZFS_LINUX_TEST_SRC([has_register_sysctl_sz], [
 		#include <linux/sysctl.h>
 	],[
-		struct ctl_table test_table[] __attribute__((unused)) = {0};
+		struct ctl_table test_table[] __attribute__((unused)) = {{}};
 		register_sysctl_sz("", test_table, 0);
 	])
 ])