mm: shrinkers: fix debugfs file permissions

The permissions for the files here are swapped as "count" is read-only and
"scan" is write-only.  While this doesn't really matter as these
permissions don't stop the files being opened for reading/writing as
appropriate, they are shown by "ls -l" and are confusing.

Link: https://lkml.kernel.org/r/20230418101906.3131303-1-john@metanate.com


Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>