btrfs-util: be more careful when invoking btrfs file system ioctls

btrfs-util: be more careful when invoking btrfs file system ioctls

If we get passed an fd that does not refer to a regular file or
directory, we should not issue btrfs ioctls on it, since it might end up
in a device driver or similar (note that DRM for example uses the same
ioctl numbers as some file system ioctls).

Hence, let's make sure to always check if something is a regular file or
directory, or is on btrfs before invoking the respective ioctls. It's
better to be safe than sorry.