mm/selftests: add a test to verify mmap_changing race with -EAGAIN

Add an unit test to verify the recent mmap_changing ABI breakage.

Note that I used some tricks here and there to make the test simple, e.g. 
I abused UFFDIO_MOVE on top of shmem with the fact that I know what I want
to test will be even earlier than the vma type check.  Rich comments were
added to explain trivial details.

Before that fix, -EAGAIN would have been written to the copy field most of
the time but not always; the test should be able to reliably trigger the
outlier case.  After the fix, it's written always, the test verifies that
making sure corresponding field (e.g.  copy.copy for UFFDIO_COPY) is
updated.

[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/20250424215729.194656-3-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>