Add dm_device_has_holders fn to to check use of the device by another device.

Add dm_device_has_holders fn to to check use of the device by another device.
Add dm_device_has_mounted_fs fn to check mounted filesystem on a device.

This requires sysfs directory to be correctly set via dm_set_sysfs_dir
(/sys by default). If sysfs dir is not used or it's set incorrectly,
dm_device_has_{holders,mounted_fs} will return 0!