Clamp tuklib_physmem() return value to SIZE_T_MAX.

On 32bit platforms it is possible to have (much) more physical RAM
than is mappable into single address space. In this case liblzma
scales the value into a request to mmap more address space than it is
theoretically possible.

Reported and tested by: pho
Reviewed by: delphij
Discussed with: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week