Use unlocked page lookup for inmem() to avoid object lock contention

[mlaier]

Summary

Let's try this again:

• make a vm_page_lookup_unlocked
• really bool-ify
• while here, export inmem alongside incore (we have a local diff for this)

[mlaier]

Moving the tinc calculation back up - needed for valid range

[markj]

sys/vm/vm_page.c

1701

I think this deserves a comment to match the others. e.g., the description for vm_page_lookup(), plus, "The page may be no longer be present in the object at the time that this function returns."

[mlaier]

Add comment per markj's request.