scsi_debug: take sdebug_host_list_lock when changing capacity

All other traversals of the sdebug_host_list take the lock.

Signed-off-by: Ewan D. Milne <emilne@redhat.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>