MFC r267213 (by alc):
Add a page size field to struct vm_page.

Approved by:	alc
MFC r268466:
Calculate the amount of resident pages by looking in the objects chain
backing the region. Add a knob to disable the residency calculation at
all.

MFC r268490:
Unconditionally initialize addr to handle the case of changed map
timestamp while the map is unlocked.

MFC r268711:
Change the calculation of the kinfo_vmentry field kve_private_resident
to reflect its name.

MFC r268712:
Followup to r268466.
- Move the code to calculate resident count into separate function.
  It reduces the indent level and makes the operation of
  vmmap_skip_res_cnt tunable more clear.
- Optimize the calculation of the resident page count for map entry.
  Skip directly to the next lowest available index and page among the
  whole shadow chain.
- Restore the use of pmap_incore(9), only to verify that current
  mapping is indeed superpage.
- Note the issue with the invalid pages.