os/bluestore: fix bluestore_wal_transaction_t encoding test

bluestore_wal_op_t::op is encoded/dumped but it is not
initialized in its ctor. so set it to 0 in its ctor. 0 is not an
valid op though, but it helps with debugging and testing.

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/os/bluestore/bluestore_types.h

@@ -258,7 +258,7 @@ struct bluestore_wal_op_t {
     OP_COPY = 2,
     OP_ZERO = 4,
   } type_t;
-  __u8 op;
+  __u8 op = 0;
   bluestore_extent_t extent;
   bluestore_extent_t src_extent;
   uint64_t src_rmw_head, src_rmw_tail;