devmapper: Fix major/minor extraction

The way devices are mapped is a bit more complex than before.
This implements the method from new_decode_dev in:
https://github.com/mirrors/linux-2.6/blob/master/include/linux/kdev_t.h

Which is what is needed on kernels > 2.6
devmapper: Fix prefix name to work with udev

Udev escapes "," used in device names to 0\x2c which breaks libdevmapper.
Instead use : to escape minor and minor which works.