amd64 pmap: do not panic on inability to insert ptp into trie

When pmap_enter_pde() needs to destroy existing kernel superpage
mapping, do not remove saved pt page from the pm_root trie. Then the
pt page does not need to be re-inserted into the trie.

If the kernel region is not mapped with the superpage, try to insert the
pt page into pm_root trie before clearing ptes. If failed, we can
return failure without a need to rewind.

Suggested by: alc
Reviewed by: alc, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D50970