btrfs-progs: Fix malloc size for superblock.

recover_prepare() in chunk-recover.c alloc memory which only contains
sizeof(struct btrfs_super_block). This will cause glibc malloc error
after superblock csum is calculated.

Use BTRFS_SUPER_INFO_SIZE to fix the bug.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

chunk-recover.c

@@ -1340,7 +1340,7 @@ static int recover_prepare(struct recover_control *rc, char *path)
 		return -1;
 	}
 
-	sb = malloc(sizeof(struct btrfs_super_block));
+	sb = malloc(BTRFS_SUPER_INFO_SIZE);
 	if (!sb) {
 		fprintf(stderr, "allocating memory for sb failed.\n");
 		ret = -ENOMEM;