mm: gup: make fault_in_safe_writeable() use fixup_user_fault()

Instead of using GUP, make fault_in_safe_writeable() actually force a
'handle_mm_fault()' using the same fixup_user_fault() machinery that
futexes already use.

Using the GUP machinery meant that fault_in_safe_writeable() did not do
everything that a real fault would do, ranging from not auto-expanding
the stack segment, to not updating accessed or dirty flags in the page
tables (GUP sets those flags on the pages themselves).

The latter causes problems on architectures (like s390) that do accessed
bit handling in software, which meant that fault_in_safe_writeable()
didn't actually do all the fault handling it needed to, and trying to
access the user address afterwards would still cause faults.

Reported-and-tested-by: Andreas Gruenbacher <agruenba@redhat.com>
Fixes: cdd591fc ("iov_iter: Introduce fault_in_iov_iter_writeable")
Link: https://lore.kernel.org/all/CAHc6FU5nP+nziNGG0JAF1FUx-GV7kKFvM7aZuU_XD2_1v4vnvg@mail.gmail.com/
Acked-by: Da...