btrfs-progs: check/original: Fix uninitialized memory for newly allocated data_backref

[BUG]
Valgrind reports the following error for fsck/002 (which only supports
original mode):
  ==97088== Conditional jump or move depends on uninitialised value(s)
  ==97088==    at 0x15BFF6: add_data_backref (main.c:4884)
  ==97088==    by 0x16025C: run_next_block (main.c:6452)
  ==97088==    by 0x165539: deal_root_from_list (main.c:8471)
  ==97088==    by 0x166040: check_chunks_and_extents (main.c:8753)
  ==97088==    by 0x166441: do_check_chunks_and_extents (main.c:8842)
  ==97088==    by 0x169D13: cmd_check (main.c:10324)
  ==97088==    by 0x11CDC6: cmd_execute (commands.h:125)
  ==97088==    by 0x11D712: main (btrfs.c:386)

[CAUSE]
In alloc_data_backref(), only ref->node is set to 0.
While ref->disk_bytenr is not initialized at all.

And then in add_data_backref(), if @back is a newly allocated data
backref, we use the garbage from back->disk_bytenr to determine if we
should reset them.

[FIX]
Fix it by initialize the whole data_backref structure in
alloc_data_backref().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

check/main.c

@@ -4516,7 +4516,7 @@ static struct data_backref *alloc_data_backref(struct extent_record *rec,
 
 	if (!ref)
 		return NULL;
-	memset(&ref->node, 0, sizeof(ref->node));
+	memset(ref, 0, sizeof(*ref));
 	ref->node.is_data = 1;
 
 	if (parent > 0) {