btrfs: ref-verify: Remove unused parameter from walk_up_tree() to kill warning

With gcc-4.1.2:

    fs/btrfs/ref-verify.c: In function ‘btrfs_build_ref_tree’:
    fs/btrfs/ref-verify.c:1017: warning: ‘root’ is used uninitialized in this function

The variable is indeed passed uninitialized, but it is never used by the
callee.  However, not all versions of gcc are smart enough to notice.

Hence remove the unused parameter from walk_up_tree() to silence the
compiler warning.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David Sterba <dsterba@suse.com>