ismounted fix

ismounted fix

ismounted handles both find-by-dev and find-by-mnt, but there's two issues:
1. for find-by-dev, it use readlink to get the canonical dev name, but
   lvm is different with other devices, the canonical name for lvm devices
   are symlinks like /dev/mapper/vg-lv00
2. for nfs mounting, just use [ -b $dev ] is not enough, it need being handled
   seperately.

Per Karel Zak's suggestion, findmnt util is suitable for this purpose, it
handles these cases well, so just use findmnt instead of implement all the
logic by ourselves. Thanks, Karel.

Signed-off-by: Dave Young <dyoung@redhat.com>