Hibernation concurrency handling is currently delegated to user.c, where it's also used for regulating the access to the snapshot device. In the prospective of making user.c a separate configuration option, such mutual exclusion is brought into hibernate.c and made available through accessor helpers hereby introduced. Signed-off-by: Domenico Andreoli <domenico.andreoli@linux.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>