PM / Hibernate: Refactor and simplify hibernation_snapshot() code

The goto statements in hibernation_snapshot() are a bit complex.
Refactor the code to remove some of them, thereby simplifying the
implementation.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>