tools/testing/selftests: add tests for !tgt, src mremap() merges

tools/testing/selftests: add tests for !tgt, src mremap() merges

Test that mremap()'ing a VMA into a position such that the target VMA on
merge is unfaulted and the source faulted is correctly performed.

We cover 4 cases:

    1. Previous VMA unfaulted:

                  copied -----|
                              v
            |-----------|.............|
            | unfaulted |(faulted VMA)|
            |-----------|.............|
                 prev

    target = prev, expand prev to cover.

    2. Next VMA unfaulted:

                  copied -----|
                              v
                        |.............|-----------|
                        |(faulted VMA)| unfaulted |
                        |.............|-----------|
                                          next

    target = next, expand next to cover.

    3. Both adjacent VMAs unfaulted:

                  copied -----|
                              v
            |-----------|.............|-----------|
            | unfaulted |(faulted VMA)| unfaulted |
            |-----------|.............|-----------|
                 prev                      next

    target = prev, expand prev to cover.

    4. prev unfaulted, next faulted:

                  copied -----|
                              v
            |-----------|.............|-----------|
            | unfaulted |(faulted VMA)|  faulted  |
            |-----------|.............|-----------|
                 prev                      next

    target = prev, expand prev to cover. Essentially equivalent to 3, but
    with additional requirement that next's anon_vma is the same as the
    copied VMA's.

Each of these are performed with MREMAP_DONTUNMAP set, which will cause a
KASAN assert for UAF or an assert on zero refcount anon_vma if a bug
exists with correctly propagating anon_vma state in each scenario.

Link: https://lkml.kernel.org/r/f903af2930c7c2c6e0948c886b58d0f42d8e8ba3.1767638272.git.lorenzo.stoakes@oracle.com
Fixes: 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jeongjun Park <aha310510@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yeoreum Yun <yeoreum.yun@arm.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>