btrfs-progs: filesystem: use btrfs_open_dir for btrfs filesystem command

btrfs-progs: filesystem: use btrfs_open_dir for btrfs filesystem command

We can use btrfs_open_dir() to check whether target dir is
in btrfs's mount point before open, instead of checking it in
kernel space of ioctl, and return fuzzy error message.

Before patch:
  # (/mnt/tmp is not btrfs mountpoint)
  #
  # btrfs filesystem df /mnt/tmp
  ERROR: couldn't get space info - Inappropriate ioctl for device
  ERROR: get_df failed Inappropriate ioctl for device
  #

After patch:
  # ./btrfs filesystem df /mnt/tmp
  ERROR: not btrfs filesystem: /mnt/tmp
  #

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>