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.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>