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.

Suggested and reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week