mm: add selftests for migration entries

Add some basic migration tests and in particular tests that will
stress both the pte and pmd migration entry wait paths.

Link: https://lkml.kernel.org/r/20220324014349.229253-1-apopple@nvidia.com


Signed-off-by: Alistair Popple <apopple@nvidia.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>