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

btrfs-progs: replace: use btrfs_open_dir for btrfs replace 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 replace cancel /mnt/tmp1
  ERROR: ioctl(DEV_REPLACE_CANCEL) failed on "/mnt/tmp1": Inappropriate ioctl for device
  # ./btrfs replace status /mnt/tmp1
  ERROR: ioctl(DEV_REPLACE_STATUS) failed on "/mnt/tmp1": Inappropriate ioctl for device

After patch:
  # ./btrfs replace cancel /mnt/tmp1
  ERROR: not a btrfs filesystem: /mnt/tmp1
  # ./btrfs replace status /mnt/tmp1
  ERROR: not a btrfs filesystem: /mnt/tmp1

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