quorum: validate vote threshold against num_children even if read-pattern is fifo

quorum: validate vote threshold against num_children even if read-pattern is fifo

We need to use threshold to check if too many write operation fails.
If threshold is larger than num children, we always get write error
event even if all write operations success.

Signed-off-by: Wen Congyang <[email protected]>
Message-id: 55962F72.3060003@cn.fujitsu.com
Reviewed-by: Alberto Garcia <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Max Reitz <[email protected]>