Fixes deadlock during import of ZVOLs

Small patch to fix issue https://github.com/zfsonlinux/zfs/issues/1488:

One must set_capacity() to 0 unless calling setcapacity() *after*
       you call add_disk(), according to these references:
         http://osdir.com/ml/linux.enbd.general/2002-10/msg00176.html
         http://osdir.com/ml/linux.enbd.general/2002-10/msg00177.html
         http://stackoverflow.com/questions/13518404/add-disk-hangs-on-insmod */