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/20221004193400.110155-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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