such string is later incorrectly parsed and blkid(8) returns
nonsenses. And for use-cases like
# eval $(blkid -o export /dev/sdb1)
it's also insecure.
Note that mount, udevd and blkid -p are based on low-level libblkid
API, it bypass the cache and directly read data from the devices.
The current udevd upstream does not depend on blkid(8) output at all,
it's directly linked with the library and all unsafe chars are encoded by
\x<hex> notation.