cxl/mbox: Replace NULL check with IS_ERR() after vmemdup_user()

vmemdup_user() returns an ERR_PTR() on failure. Use IS_ERR()
to check the return value.
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20220407010915.1211258-1-alison.schofield@intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/core/mbox.c

@@ -265,7 +265,7 @@ static int cxl_mbox_cmd_ctor(struct cxl_mbox_cmd *mbox,
 	if (in_size) {
 		mbox->payload_in = vmemdup_user(u64_to_user_ptr(in_payload),
 						in_size);
-		if (!mbox->payload_in)
+		if (IS_ERR(mbox->payload_in))
 			return PTR_ERR(mbox->payload_in);
 
 		if (!cxl_payload_from_user_allowed(opcode, mbox->payload_in)) {