x86/microcode: Add missing unlock in microcode_update_helper()

microcode_update_helper() may return early while holding
cpu_add_remove_lock, hence preventing any writers from taking it again.

Leave through the `put` label instead so it's properly released.

Fixes: 5ed12565aa ("microcode: rendezvous CPUs in NMI handler and load ucode")
Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: b35b22acb8
master date: 2023-06-05 16:11:10 +0100

xen/arch/x86/cpu/microcode/core.c

@@ -596,7 +596,8 @@ static long cf_check microcode_update_helper(void *data)
         printk(XENLOG_WARNING
                "CPU%u is expected to lead ucode loading (but got CPU%u)\n",
                nmi_cpu, cpumask_first(&cpu_online_map));
-        return -EPERM;
+        ret = -EPERM;
+        goto put;
     }
 
     patch = parse_blob(buffer->buffer, buffer->len);