skip updating /etc/default/grub for s390x

Resolves: bz2104534

When running "kdumpctl reset-crashkernel --kernel=ALL" on s390x,
sed: can't read /etc/default/grub: No such file or directory
sed: can't read /etc/default/grub: No such file or directory

This happens because s390x doesn't use the grub bootloader and
/etc/default/grub doesn't exist.

Reported-by: smitterl@redhat.com
Reviewed-by: Philipp Rudo <prudo@redhat.com>
Signed-off-by: Coiby Xu <coxu@redhat.com>

kdumpctl

@@ -1428,6 +1428,10 @@ _update_kernel_arg_in_grub_etc_default()
 {
 	local _para=$1 _val=$2 _para_val
 
+	if [[ $(uname -m) == s390x ]]; then
+		return
+	fi
+
 	if [[ -n $_val ]]; then
 		_para_val="$_para=$_val"
 	fi