PM / Hibernate: Reduce autotuned default image size

The hibernate image size autotuning mechanism sets the default
image size to 5/2 of the total system RAM, but it is reported
that on some systems device drivers allocate substantial
amounts of memory during suspend and the creation of the image
fails as a result (too little memory is preallocated).

Modify the autotuning mechanism to use 1/3 instead of 2/5 of RAM
as the default image size, which is reported to be sufficient for
the affected systems.

References: https://bugzilla.kernel.org/show_bug.cgi?id=30482


Reported-and-tested-by: Martin Steigerwald <Martin@Lichtvoll.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>