rbd: don't over-allocate space for object prefix

In rbd_header_from_disk() the object prefix buffer is sized based on
the maximum size it's block_name equivalent on disk could be.

Instead, only allocate enough to hold null-terminated string from
the on-disk header--or the maximum size of no NUL is found.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>

drivers/block/rbd.c

@@ -519,18 +519,19 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 				 struct rbd_image_header_ondisk *ondisk)
 {
 	u32 snap_count;
+	size_t len;
 	size_t size;
 
 	memset(header, 0, sizeof (*header));
 
 	snap_count = le32_to_cpu(ondisk->snap_count);
 
-	size = sizeof (ondisk->object_prefix) + 1;
-	header->object_prefix = kmalloc(size, GFP_KERNEL);
+	len = strnlen(ondisk->object_prefix, sizeof (ondisk->object_prefix));
+	header->object_prefix = kmalloc(len + 1, GFP_KERNEL);
 	if (!header->object_prefix)
 		return -ENOMEM;
-	memcpy(header->object_prefix, ondisk->object_prefix, size - 1);
-	header->object_prefix[size - 1] = '\0';
+	memcpy(header->object_prefix, ondisk->object_prefix, len);
+	header->object_prefix[len] = '\0';
 
 	if (snap_count) {
 		header->snap_names_len = le64_to_cpu(ondisk->snap_names_len);