On Linux systems that have statx available, stat_dev() returned
stx_dev_major, which could be the same value for different partitions.
Change it to return makedev(stx_dev_major, stx_dev_minor) to get a unique
ID (meaning a unique UDSEntry::UDS_DEVICE_ID).
Add a unit test to check that UDS_DEVICE_ID is unique for each individual
partition.