mm: fix invalid page pointer returned with FOLL_PIN gups

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2037300

Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=43231677

Upstream status: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git

This patch fixes the host crash on booting VM with vfio device assigned.

I only picked the real fix out of the 5 patches in the patchset (which is a one liner but the real fix to the problem), the rest 4 patches are pure cleanups and can be picked up later. I'm keeping the changeset small just in case we'll hit the blocker phase.

ITM26 is the end of this week, it'll be much appreciated if this can be reviewed before Thursday (Feb 24), which is still pre-blocker phase.

Please review. Thanks in advance!

Signed-off-by: Peter Xu peterx@redhat.com