PM / Hibernate: Move the first_sector out of swsusp_write

The first sector knowledge is swap-only specific. Move it into the
swap handle. This will be needed for later non-swap specific code
moving into snapshot.c.

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