mem: Update soft-dirty detection to work on -mm tree

The soft-dirty API has changed slightly -- now the bit in
question _is_ in pagemap file (not pagemap2) but to see it
we have to reset soft-dirty for anyone first.

Teach the kerndat soft-dirty checker this fact. The actual
pagemap reading code already knows select pagemap/pagemap2
file itself.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>