mem: check if allocation size is too big

mem: check if allocation size is too big

Mapping size is a 64-bit integer, but mmap() will accept size_t for
size mappings. A user could request a mapping with an alignment, which
would have overflown size_t, so check if (size + alignment) will
overflow size_t.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>