selftests: mm: add a test for mutually aligned moves > PMD size

This patch adds a test case to check if a PMD-alignment optimization
successfully happens.

I add support to make sure there is some room before the source mapping,
otherwise the optimization to trigger PMD-aligned move will be disabled as
the kernel will detect that a mapping before the source exists and such
optimization becomes impossible.

Link: https://lkml.kernel.org/r/20230903151328.2981432-5-joel@joelfernandes.org


Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>