common/rc: Use -v option with findmnt

common/rc: Use -v option with findmnt

Since btrfs puts in the subvol name in the device, we should
use -v option to not print the bind mounts or btrfs volumes.

Otherwise if a new subvolume is set as the default volume to be
mounted, as what btrfs/009 does, then next _require_scratch fails

  SCRATCH_DEV=/dev/vdc is mounted but not on SCRATCH_MNT=/scratch - aborting
  Already mounted result:
  /dev/vdc[/newvol] /scratch

[eguan: update commit log a bit, add failure example]

Fixes: 5e6892d ("common/rc: use findmnt to check mounted device")
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>