PM / Hibernate: Improve comments in hibernate_preallocate_memory()

One comment in hibernate_preallocate_memory() is wrong, so fix it and
add one more comment to clarify the meaning of the fixed one.
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>

kernel/power/snapshot.c

@@ -1318,12 +1318,14 @@ int hibernate_preallocate_memory(void)
 
 	/* Compute the maximum number of saveable pages to leave in memory. */
 	max_size = (count - (size + PAGES_FOR_IO)) / 2 - 2 * SPARE_PAGES;
+	/* Compute the desired number of image pages specified by image_size. */
 	size = DIV_ROUND_UP(image_size, PAGE_SIZE);
 	if (size > max_size)
 		size = max_size;
 	/*
-	 * If the maximum is not less than the current number of saveable pages
-	 * in memory, allocate page frames for the image and we're done.
+	 * If the desired number of image pages is at least as large as the
+	 * current number of saveable pages in memory, allocate page frames for
+	 * the image and we're done.
 	 */
 	if (size >= saveable) {
 		pages = preallocate_image_highmem(save_highmem);