xen/balloon: Clear PG_offline in balloon_retrieve()

Let's move the clearing to balloon_retrieve(). In
bp_state increase_reservation(), we now clear the flag a little earlier
than before, however, this should not matter for XEN.

Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>