btrfs-progs: Avoid interpreting options after "--" when getting unit mode

btrfs-progs: Avoid interpreting options after "--" when getting unit mode

* actual result

  ======================================
  # ./btrfs device usage -- -m /btrfs

  /dev/sdf1, ID: 1
     Device size:          95367.41MiB
     Data,single:          2056.00MiB
     Metadata,DUP:         2048.00MiB
     System,DUP:             16.00MiB
     Unallocated:          91247.41MiB
  ======================================

* expected result

  ======================================
  # ./btrfs device usage -- -m /btrfs

  ERROR: can't access '-m': No such file or directory
  ======================================

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>