Add a bit of paranoia to uvm_pageinsert.

At various times diffs have had debugging that checked that we don't
insert a page into the tree on top of an existing page, leaking that
page's references.  Until the recent hackathon (and introduction if
uvm_pagealloc_multi) the bufcache for example did a rb tree look up on
insert to check (under #ifdef DEBUG || 1) so instead just check it on
pageinsert every time, since RB_INSERT returns any duplicates so this
check is pretty much free.

``emphatically yes'' beck@

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