selftests/vm: add test to measure MADV_UNMERGEABLE performance

selftests/vm: add test to measure MADV_UNMERGEABLE performance

Let's add a test to measure performance of KSM breaking not triggered via
COW, but triggered by disabling KSM on an area filled with KSM pages via
MADV_UNMERGEABLE.

Link: https://lkml.kernel.org/r/20221021101141.84170-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>