cxl/mbox: Fix missing variable payload checks in cmd size validation

The conversion of command sizes to unsigned missed a couple of checks
against variable size payloads during command validation, which made all
variable payload commands unconditionally fail. Add the checks back using
the new CXL_VARIABLE_PAYLOAD scheme.

Fixes: 26f89535 ("cxl/mbox: Use type __u32 for mailbox payload sizes")
Cc: <stable@vger.kernel.org>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Alison Schofield <alison.schofield@intel.com>
Reported-by: Abhi Cs <abhi.cs@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/20220628220109.633564-1-vishal.l.verma@intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/core/mbox.c

@@ -355,11 +355,13 @@ static int cxl_to_mem_cmd(struct cxl_mem_command *mem_cmd,
 		return -EBUSY;
 
 	/* Check the input buffer is the expected size */
-	if (info->size_in != send_cmd->in.size)
+	if ((info->size_in != CXL_VARIABLE_PAYLOAD) &&
+	    (info->size_in != send_cmd->in.size))
 		return -ENOMEM;
 
 	/* Check the output buffer is at least large enough */
-	if (send_cmd->out.size < info->size_out)
+	if ((info->size_out != CXL_VARIABLE_PAYLOAD) &&
+	    (send_cmd->out.size < info->size_out))
 		return -ENOMEM;
 
 	*mem_cmd = (struct cxl_mem_command) {