PM / hibernate: Documentation: Fix script for unswapping

System can have mmaped also character devices (e.g dri devices by X) or deleted
files. Running cat on character devices is really bad idea (system can hang) so
run cat only on regular files. Also mmaped files can have spaces in filenames.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
[rjw: Subject]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>