PM: hibernate: Fix snapshot partial write lengths

snapshot_write() is inappropriately limiting the amount of data that can
be written in cases where a partial page has already been written. For
example, one would expect to be able to write 1 byte, then 4095 bytes to
the snapshot device, and have both of those complete fully (since now
we're aligned to a page again). But what ends up happening is we write 1
byte, then 4094/4095 bytes complete successfully.

The reason is that simple_write_to_buffer()'s second argument is the
total size of the buffer, not the size of the buffer minus the offset.
Since simple_write_to_buffer() accounts for the offset in its
implementation, snapshot_write() can just pass the full page size
directly down.
Signed-off-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/user.c

@@ -177,7 +177,7 @@ static ssize_t snapshot_write(struct file *filp, const char __user *buf,
 		if (res <= 0)
 			goto unlock;
 	} else {
-		res = PAGE_SIZE - pg_offp;
+		res = PAGE_SIZE;
 	}
 
 	if (!data_of(data->handle)) {