i386: Use atomic 64bit load to read PDE value from PAE pagetables in
pmap_kextract().

pmap_kextract() can race with promotion/demotion on the kernel page
table, in which case current non-atomic 64bit read would see torn
value, breaking pmap_kextract(). pmap_kextract() would correctly
handle either promoted or demoted PDE, but not a mix where one word
is from a different state.

It requires PAE and > 4G memory to reproduce. We observed this in
real loads, both for intensive use of malloc(9)/free(9) where
vtoslab() returned invalid pointer to the slab, and with the use of
busdma_bounce, where incorrect page was bounced.

In collaboration with: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18714