Sector-based blk_write() should die; convert the one-off
variant blk_write_zeroes() to use an offset/count interface
instead.  Likewise for blk_co_write_zeroes() and
blk_aio_write_zeroes().
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>