selftests: mm: gup_longterm: test unsharing logic when R/O pinning

In our FOLL_LONGTERM tests, we prefault the page tables for the GUP-fast
test cases to be able to find a PTE and exercise the "longterm pinning
allowed" logic on the GUP-fast path where possible.

For now, we always prefault the page tables writable, resulting in PTEs
that are writable.

Let's cover more cases to also test if our unsharing logic works as
expected (and is able to make progress when there is nothing to unshare)
by mprotect'ing the range R/O when R/O-pinning, so we don't get PTEs that
are writable.

This change would have found an issue introduced by commit a12083d721d7
("mm/gup: handle hugepd for follow_page()"), whereby R/O pinning was not
able to make progress in all cases, because unsharing logic was not
provided with the VMA to decide at some point that long-term R/O pinning a
!anon page is fine.

Link: https://lkml.kernel.org/r/20240430131508.86924-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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