vm_fault: avoid vm_page_next()

[dougm]

Summary

vm_fault calls vm_page_next in two places where it ought to use TAILQ_NEXT and add assertions. The result is that in a NODEBUG kernel, the vm_page_next code does needless pindex calculations, and in a GENERIC kernel it leads either to a failed KASSERT or a null-pointer reference that could be caught more clearly by the KASSERTs I've added.