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

btrfs-progs: fragments: use btrfs_open_dir for btrfs-fragments 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
deeper code, and return fuzzy error message.

Before patch:
  ./btrfs-fragments -o 123 /mnt/tmp1
  ERROR: can't perform the search

After patch:
  # ./btrfs-fragments -o 123 /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>