Do not use vmspace_resident_count() for the OOM process selection.
Residency count track the number of pte entries installed into the
current pmap, which does not reflect the consumption of the physical
memory by the address map. Due to several mechanisms like pv entries
reclamation, copy on write etc. the resident pte entries count may be
much less than the amount of physical memory kept by the process.

Provide the OOM-specific vm_pageout_oom_pagecount() function which
estimates the amount of reclamaible memory which could be stolen if
the process is killed.

Reported and tested by: pho
Reviewed by: alc
Comments text by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks