cxl/mbox: Add a check on input payload size

A bug in the LSA code resulted in transfers slightly larger
than the mailbox size. Let us make it easier to catch similar
issues in future by adding a low level check.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220815154044.24733-2-Jonathan.Cameron@huawei.com


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