Btrfs, raid56: use a variant to record the operation type

We will introduce new operation type later, if we still use integer
variant as bool variant to record the operation type, we would add new
variant and increase the size of raid bio structure. It is not good,
by this patch, we define different number for different operation,
and we can just use a variant to record the operation type.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>