Introduce EXT4_BLOCK_ZERO macro to initialize ext4_block

lwext4/ext4_bcache.h

@@ -42,6 +42,8 @@
 #include <stdint.h>
 #include <stdbool.h>
 
+#define EXT4_BLOCK_ZERO() {.dirty = 0, .lb_id = 0, .cache_id = 0, .data = 0}
+
 /**@brief   Single block descriptor*/
 struct ext4_block {
 	/**@brief   Dirty flag*/