We actually need one byte less (next_mb_id is exclusive, first_mb_id is
inclusive). While at it, compact the code.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201112133815.13332-3-david@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>