common/rc: improve dev mounted check helper

common/rc: improve dev mounted check helper

There is a problem of missing fstype check in _is_mounted() helper,
it will return the mountpoint if only the device arguments matches.

For example:
  Base mounted filesystem:
    /dev/sda2 on /boot type ext4 (rw,relatime,data=ordered)

  FSTYPE=xfs
  mountpoint=`_is_mounted /dev/sda1`
  echo "$mountpoint"

  Output: /boot

This patch rename _is_mounted to _is_dev_mounted because it check
the given device only (not mount dir), and add an optional "fstype"
parameter, let user specify file system type instead of default
FSTYPE. Finally, use findmnt instead of mount to avoid complex
processing of mount info and fix this problem simply.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>