Add "external" bitmap support to blockstore

src/blockstore.h

@@ -64,6 +64,10 @@ Input:
 - offset, len = offset and length within object. length may be zero, in that case
   read operation only returns the version / write operation only bumps the version
 - buf = pre-allocated buffer for data (read) / with data (write). may be NULL if len == 0.
+- bitmap = <entry_attr_size> bytes long arbitrary data stored for each object in the metadata area.
+  when <entry_attr_size> fits into pointer size, it should be passed as this field's value.
+  when it doesn't fit, this field should be a pointer to that piece of data.
+  named "bitmap" because it's used for the "external bitmap" in Vitastor.
 
 Output:
 - retval = number of bytes actually read/written or negative error number (-EINVAL or -ENOSPC)
@@ -141,6 +145,7 @@ struct blockstore_op_t
     uint32_t offset;
     uint32_t len;
     void *buf;
+    void *bitmap;
     int retval;
 
     uint8_t private_data[BS_OP_PRIVATE_DATA_SIZE];