mm/selftest: uffd: explain the write missing fault check

It's not obvious why we had a write check for each of the missing
messages, especially when it should be a locking op.  Add a rich comment
for that, and also try to explain its good side and limitations, so that
if someone hit it again for either a bug or a different glibc impl
there'll be some clue to start with.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Nadav Amit <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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