selftests/proc: fixup proc-empty-vm test after KSM changes

/proc/${pid}/smaps_rollup is not empty file even if process's address
space is empty, update the test.

Link: https://lkml.kernel.org/r/725e041f-e9df-4f3d-b267-d4cd2774a78d@p183


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>