fs/proc/task_mmu: report SOFT_DIRTY bits through the PAGEMAP_SCAN ioctl

The PAGEMAP_SCAN ioctl returns information regarding page table entries. 
It is more efficient compared to reading pagemap files.  CRIU can start to
utilize this ioctl, but it needs info about soft-dirty bits to track
memory changes.

We are aware of a new method for tracking memory changes implemented in
the PAGEMAP_SCAN ioctl.  For CRIU, the primary advantage of this method is
its usability by unprivileged users.  However, it is not feasible to
transparently replace the soft-dirty tracker with the new one.  The main
problem here is userfault descriptors that have to be preserved between
pre-dump iterations.  It means criu continues supporting the soft-dirty
method to avoid breakage for current users.  The new method will be
implemented as a separate feature.

[[email protected]: update tools/include/uapi/linux/fs.h]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrei Vagin <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Cc: Michał Mirosław <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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