misc: pci_endpoint_test: Prevent some integer overflows

"size + max" can have an arithmetic overflow when we're allocating:

  orig_src_addr = dma_alloc_coherent(dev, size + alignment, ...

Add a few checks to prevent that.

Fixes: 13107c60 ("misc: pci_endpoint_test: Add support to provide aligned buffer addresses")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>