btrfs: fix gcc-4.8 build warning for struct initializer

Some older compilers like gcc-4.8 warn about mismatched curly braces in
a initializer:

fs/btrfs/backref.c: In function 'is_shared_data_backref':
fs/btrfs/backref.c:394:9: error: missing braces around
initializer [-Werror=missing-braces]
  struct prelim_ref target = {0};
         ^
fs/btrfs/backref.c:394:9: error: (near initialization for
'target.rbnode') [-Werror=missing-braces]

Use the GNU empty initializer extension to avoid this.

Fixes: ed58f2e ("btrfs: backref, don't add refs from shared block when resolving normal backref")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -391,7 +391,7 @@ static int is_shared_data_backref(struct preftrees *preftrees, u64 bytenr)
 	struct rb_node **p = &preftrees->direct.root.rb_root.rb_node;
 	struct rb_node *parent = NULL;
 	struct prelim_ref *ref = NULL;
-	struct prelim_ref target = {0};
+	struct prelim_ref target = {};
 	int result;
 
 	target.parent = bytenr;