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: ed58f2e6

 ("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>