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: 26f89535a5bb ("cxl/mbox: Use type __u32 for mailbox payload sizes")
Cc: <[e-mail]>
Cc: Ira Weiny <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Alison Schofield <[e-mail]>
Reported-by: Abhi Cs <[e-mail]>
Reviewed-by: Dan Williams <[e-mail]>
Reviewed-by: Alison Schofield <[e-mail]>
Signed-off-by: Vishal Verma <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

drivers/cxl/core/mbox.c

···
 		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) {