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

btrfs-progs: inspect: use btrfs_open_dir for btrfs inspect 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 inspect-internal rootid /mnt/tmp1
  ERROR: Failed to lookup root id - Inappropriate ioctl for device
  btrfs inspect-internal rootid: rootid failed with ret=-1
  # ./btrfs inspect-internal inode-resolve 256 /mnt/tmp1
  ioctl ret=-1, error: Inappropriate ioctl for device
  # ./btrfs inspect-internal min-dev-size /mnt/tmp1
  Error invoking tree search ioctl: Inappropriate ioctl for device

After patch:
  # ./btrfs inspect-internal rootid /mnt/tmp1
  ERROR: not a btrfs filesystem: /mnt/tmp1
  # ./btrfs inspect-internal inode-resolve 256 /mnt/tmp1
  ERROR: not a btrfs filesystem: /mnt/tmp1
  # ./btrfs inspect-internal min-dev-size /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>