Address the case 2b version of inconsistent view across threads of

Address the case 2b version of inconsistent view across threads of
a page undergoing copy-on-write faulting.  We fixed the case 1b
version in rev 1.125 (2022-02-01), but missed this other path.

jsg@ noted that in NetBSD Chuck Silvers had a relevant commit, their
rev 1.234 (2023-08-13), which looks like it fixed both cases due
to their refactoring of common code into a uvmfault_promote()
function.

ok mpi@ jca@