btrfs-progs: du: fix to skip not btrfs dir/file

btrfs-progs: du: fix to skip not btrfs dir/file

'btrfs file du' is a very useful tool to watch my system
file usage information with snapshot aware.

when trying to run following commands:
[root@localhost btrfs-progs]# btrfs file du /
     Total   Exclusive  Set shared  Filename
ERROR: Failed to lookup root id - Inappropriate ioctl for device
ERROR: cannot check space of '/': Unknown error -1

and My Filesystem looks like this:
[root@localhost btrfs-progs]# df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
devtmpfs       devtmpfs   16G     0   16G   0% /dev
tmpfs          tmpfs      16G  368K   16G   1% /dev/shm
tmpfs          tmpfs      16G  1.4M   16G   1% /run
tmpfs          tmpfs      16G     0   16G   0% /sys/fs/cgroup
/dev/sda3      btrfs      60G   19G   40G  33% /
tmpfs          tmpfs      16G  332K   16G   1% /tmp
/dev/sdc       btrfs     2.8T  166G  1.7T   9% /data
/dev/sda2      xfs       2.0G  452M  1.6G  23% /boot
/dev/sda1      vfat      1.9G   11M  1.9G   1% /boot/efi
tmpfs          tmpfs     3.2G   24K  3.2G   1% /run/user/1000

So I installed Btrfs as my root partition, but boot partition
can be other fs.

We can Let btrfs tool aware of this is not a btrfs file or
directory and skip those files, so that someone like me
could just run 'btrfs file du /' to scan all btrfs filesystems.

After patch, it will look like:
   Total   Exclusive  Set shared  Filename
     0.00B       0.00B           -  //root/.bash_logout
     0.00B       0.00B           -  //root/.bash_profile
     0.00B       0.00B           -  //root/.bashrc
     0.00B       0.00B           -  //root/.cshrc
     0.00B       0.00B           -  //root/.tcshrc

This works for me to analysis system usage and analysis
performaces.

Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>