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

btrfs-progs: subvolume: use btrfs_open_dir for btrfs subvolume 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 subvolume create /mnt/tmp/123
  Create subvolume '/mnt/tmp/123'
  ERROR: cannot create subvolume - Inappropriate ioctl for device
  #

After patch:
  # btrfs subvolume create /mnt/tmp/123
  ERROR: not btrfs filesystem: /mnt/tmp
  #

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