amd64: fix user pt_page accounting

[alc]

Summary

When a superpage mapping is destroyed and the original page table page containing 4KB mappings that was being held in reserve is deallocated, the pt_page count was not being decremented. Consequentially, the user pt_count would grow over time.

Test Plan

For example, after multiple iterations of "buildworld", I was seeing the following illogical counts:

vm.pmap.kernel_pt_page_count: 2184
vm.pmap.user_pt_page_count: 2280849
vm.pmap.pv_page_count: 106

With this change, I now see:

vm.pmap.kernel_pt_page_count: 2183
vm.pmap.user_pt_page_count: 344
vm.pmap.pv_page_count: 105