PM/Hibernate: Move memory shrinking to snapshot.c (rev. 2)

A future patch is going to modify the memory shrinking code so that
it will make memory allocations to free memory instead of using an
artificial memory shrinking mechanism for that.  For this purpose it
is convenient to move swsusp_shrink_memory() from
kernel/power/swsusp.c to kernel/power/snapshot.c, because the new
memory-shrinking code is going to use things that are local to
kernel/power/snapshot.c .

[rev. 2: Make some functions static and remove their headers from
 kernel/power/power.h]

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Wu Fengguang <fengguang.wu@intel.com>