amd64 pmap: preallocate another page table page in pmap_demote_DMAP()

To avoid a possible panic in pmap_demote_DMAP(), preallocate the page
table page that may be needed by pmap_demote_pde() before acquiring the
kernel pmap lock, so that we can wait until the allocation succeeds.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51091