PM/Hibernate: Rename disk.c to hibernate.c

Change the name of kernel/power/disk.c to kernel/power/hibernate.c
in analogy with the file names introduced by the changes that
separated the suspend to RAM and standby funtionality from the
common PM functions.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>