btrfs-progs: use btrfs_open_dir for btrfs device command

btrfs-progs: use btrfs_open_dir for btrfs device command

We can use btrfs_open_dir() to check whether the target dir is
under btrfs mount point before openning, instead of checking it in kernel
through ioctl, and returning a  fuzzy error message.

Before patch:
  # (/mnt/tmp is not btrfs mountpoint)
  #
  # btrfs device add -f /dev/sda13 /mnt/tmp
  ERROR: error adding the device '/dev/sda13' - Inappropriate ioctl for device
  #

After patch:
  # btrfs device add -f /dev/sda13 /mnt/tmp
  ERROR: not btrfs filesystem: /mnt/tmp
  #

Similar fix for device remove and device usage.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
[renamed to btrfs_open_dir]
Signed-off-by: David Sterba <dsterba@suse.com>