lib/canonicalize: verify DM paths [coverity scan]

lib/canonicalize: verify DM paths [coverity scan]

Now the code only checks that /sys/.../dm/name exists, but never
verify the device node in /dev (because path prefix is never NULL).

The prefix is used to redirect hardcoded paths to /sys dumps (e.g.
lsblk regression tests, etc.)

This bug has been introduced in v2.33. Fortunately, it's probably no
big issue as /dev is always in sync with /sys (thanks to udevd).

Signed-off-by: Karel Zak <kzak@redhat.com>