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

btrfs-progs: balance: use btrfs_open_dir for btrfs balance 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:
  # btrfs balance start /mnt/tmp
  ERROR: error during balancing '/mnt/tmp' - Inappropriate ioctl for device
  There may be more info in syslog - try dmesg | tail
  #

After patch:
  # btrfs balance start /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>