pmap_growkernel() on large memory machines. at this point we've already
called uvm_page_init(), so we can no longer use uvm_page_physget().
but the flag we use to decide whether to use uvm_page_physget() vs.
uvm_pagealloc() is only set at the end of pmap_init(), so we use
uvm_page_physget() anyway. switch to using uvm.page_init_done to decide
how to allocate pages, since that's the real indicator of when we have to
change methods.
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