cxl/pmem: Use size_add() against integer overflow

"struct_size() + n" may cause a integer overflow,
use size_add() to handle it.

Signed-off-by: Yu Zhe <yuzhe@nfschina.com>
Link: https://lore.kernel.org/r/20220927070247.23148-1-yuzhe@nfschina.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>