Final fix for the lack of zeroing out of old metadata entries

If a crash occurs during flushing a redirect-write it may happen so that
the disk contains both old and new metadata entries. This is OK, but prior
to 0.8.0 after this situation OSDs started without problem, but then they
crashed after some more overwrites with a "tried to overwrite non-zero
metadata entry" error. 0.8.0 introduced a change that was intended to fix
this situation, but rather than fixing it it prevented OSDs from starting,
now because of a "big_write journal_entry was allocated over another object"
error... :-)

This change finally fixes the original issue.

Followup to 54ef2c389f

src/blockstore_init.h

@@ -24,7 +24,10 @@ class blockstore_init_meta
     uint64_t md_offset = 0;
     uint64_t next_offset = 0;
     uint64_t entries_loaded = 0;
-    bool handle_entries(uint8_t *buf, uint64_t count, uint64_t done_cnt);
+    unsigned entries_per_block = 0;
+    int i = 0, j = 0;
+    std::vector<uint64_t> entries_to_zero;
+    bool handle_meta_block(uint8_t *buf, uint64_t count, uint64_t done_cnt);
     void handle_event(ring_data_t *data, int buf_num);
 public:
     blockstore_init_meta(blockstore_impl_t *bs);